Workaround g_utf8_find_next_char() not incrementing past NUL char (#777973)

If PipeCapture reads a NUL character, a valid UTF-8 character, it causes
GParted to allocate all available memory and crash.  The while loop in
PipeCapture::OnReadable() loops forever reading the same NUL character
from readbuf because g_utf8_find_next_char() doesn't advance past it.
Hence an infinite number of NUL characters are added to the current
line, linevec.

Workaround this by checking for this failure case of
g_utf8_find_next_char() and increment past the NUL character.

This is actually a bug recently fixed in glib 2.49.3 released
2016-07-17.  References:

*   Bug 547200 - g_utf8_find_next_char() issues
    https://bugzilla.gnome.org/show_bug.cgi?id=547200

*   https://git.gnome.org/browse/glib/commit/?id=e0e652e4032a181d4f0b0a12aeddf0678b7a3c04
    Fix a corner-case in g_utf8_find_next_char

    In the case that *p is '\0', we should return p + 1, not p.
    This change allows to simplify g_utf8_find_next_char a bit.

Bug 777973 - Segmentation fault on bad disk

src/PipeCapture.cc

@@ -144,7 +144,13 @@ bool PipeCapture::OnReadable( Glib::IOCondition condition )
 			else
 			{
 				// Advance read pointer past the read UTF-8 character.
-				read_ptr = g_utf8_find_next_char( read_ptr, end_ptr );
+				const char * new_ptr = g_utf8_find_next_char( read_ptr, end_ptr );
+				if ( new_ptr == read_ptr && *read_ptr == '\0' )
+					// Workaround bug in g_utf8_find_next_char() which
+					// stops it advancing past NUL char in buffer
+					// delimited by an end pointer.
+					new_ptr ++;
+				read_ptr = new_ptr;
 				if ( read_ptr == NULL )
 					read_ptr = end_ptr;
 			}