Refactor ::OnReadable() creating get_utf8_char_validated() (#136)

Extract call to GLib's g_utf8_get_char_validated() and the associated
workaround to also read NUL characters into a separate function to make
PipeCapture::OnReadable() a little smaller and simpler, so easier to
understand.

Add max_len > 0 clause into get_utf8_char_validated() like this:
    if (uc == UTF8_PARTIAL && max_len > 0)
so that the NUL character reading workaround is only applied when
max_len specifies the maximum number of bytes to read, rather than
when -1 specifies reading a NUL termination string.  This makes
get_utf8_char_validated() a complete wrapper of
g_utf8_get_char_validated() [1], even though GParted always specifies
the maximum number of bytes to read.

No longer describe the inability to read NUL characters as a bug [2]
since the GLib author's said it wasn't [3].

[1] GLib Reference Manual, Unicode Manipulation Functions,
    g_utf8_get_char_validated ()
    https://developer.gnome.org/glib/stable/glib-Unicode-Manipulation.html#g-utf8-get-char-validated

[2] 8dbbb47ce2
    Workaround g_utf8_get_char_validate() bug with embedded NUL bytes
    (#777973)

[3] Bug 780095 - g_utf8_get_char_validated() stopping at nul byte even
    for length specified buffers
    https://bugzilla.gnome.org/show_bug.cgi?id=780095#18
        "If g_utf8_get_char_validated() encounters a nul byte in the
        middle of a string of given longer length, it returns -2,
        indicating a partial gunichar.  That is not the obvious
        behaviour, but since g_utf8_get_char_validated() has been API
        for a long time, the behaviour cannot be changed.
        "

Closes #136 - 1.2.0: test suite is failing in test_PipeCapture

include/PipeCapture.h

@@ -45,6 +45,7 @@ private:
 	                             gpointer data );
 	static void append_unichar_vector_to_utf8( std::string & str,
 	                                           const std::vector<gunichar> & ucvec );
+	static gunichar get_utf8_char_validated(const char *p, gssize max_len);
 	static int utf8_char_length( unsigned char firstbyte );
 
 	Glib::RefPtr<Glib::IOChannel> channel;  // Wrapper around fd

src/PipeCapture.cc

@@ -30,6 +30,11 @@ namespace GParted {
 
 const size_t READBUF_SIZE = 64*KIBIBYTE;
 
+
+const gunichar UTF8_PARTIAL = (gunichar)-2;
+const gunichar UTF8_INVALID = (gunichar)-1;
+
+
 PipeCapture::PipeCapture( int fd, Glib::ustring &buffer ) : fill_offset( 0 ),
                                                             cursor( 0 ),
                                                             line_start( 0 ),
@@ -125,21 +130,7 @@ bool PipeCapture::OnReadable( Glib::IOCondition condition )
 		fill_offset = 0;
 		while ( read_ptr < end_ptr )
 		{
-			const gunichar UTF8_PARTIAL = (gunichar)-2;
+			gunichar uc = get_utf8_char_validated(read_ptr, end_ptr - read_ptr);
-			const gunichar UTF8_INVALID = (gunichar)-1;
-			gunichar uc = g_utf8_get_char_validated( read_ptr, end_ptr - read_ptr );
-			if ( uc == UTF8_PARTIAL )
-			{
-				// Workaround bug in g_utf8_get_char_validated() in which
-				// it reports an partial UTF-8 char when a NUL byte is
-				// encountered in the middle of a multi-byte character,
-				// yet there are more bytes available in the length
-				// specified buffer.  Report as invalid character instead.
-				int len = utf8_char_length( *read_ptr );
-				if ( len == -1 || read_ptr + len <= end_ptr )
-					uc = UTF8_INVALID;
-			}
-
 			if ( uc == UTF8_PARTIAL )
 			{
 				// Partial UTF-8 character at end of read buffer.  Copy to
@@ -258,6 +249,29 @@ void PipeCapture::append_unichar_vector_to_utf8( std::string & str, const std::v
 	}
 }
 
+
+// GLib's g_utf8_get_char_validated() always considers strings as being NUL terminated,
+// even when max_len is specified, hence can't read NUL characters.  This wrapper can read
+// NUL characters when max_len is specified.
+// Reference:
+//     https://developer.gnome.org/glib/stable/glib-Unicode-Manipulation.html#g-utf8-get-char-validated
+gunichar PipeCapture::get_utf8_char_validated(const char *p, gssize max_len)
+{
+	gunichar uc = g_utf8_get_char_validated(p, max_len);
+	if (uc == UTF8_PARTIAL && max_len > 0)
+	{
+		// If g_utf8_get_char_validated() found a NUL byte in the middle of a
+		// multi-byte character, even when there are more bytes available as
+		// specified by max_len, it reports a partial UTF-8 character.  Report
+		// this case as an invalid character instead.
+		int len = utf8_char_length(*p);
+		if (len == -1 || (gssize)len <= max_len)
+			uc = UTF8_INVALID;
+	}
+	return uc;
+}
+
+
 int PipeCapture::utf8_char_length( unsigned char firstbyte )
 {
 	// Recognise the size of FSS-UTF (1992) / UTF-8 (1993) characters given the first