Move initial clearing of output capture buffers into PipeCapture class (#777973)

Seems more logical to initially clear the output capture buffer in a
single location in the PipeCapture class which reads the command output
into said buffer, rather than each calling site before the PipeCapture
objects are constructed.

Bug 777973 - Segmentation fault on bad disk

src/FileSystem.cc

@@ -133,8 +133,6 @@ int FileSystem::execute_command_internal( const Glib::ustring & command, Operati
 	fcntl( out, F_SETFL, O_NONBLOCK );
 	fcntl( err, F_SETFL, O_NONBLOCK );
 	Glib::signal_child_watch().connect( sigc::mem_fun( *this, &FileSystem::store_exit_status ), pid );
-	output.clear();
-	error.clear();
 	pipecount = 2;
 	PipeCapture outputcapture( out, output );
 	PipeCapture errorcapture( err, error );

src/PipeCapture.cc

@@ -25,6 +25,7 @@ namespace GParted {
 PipeCapture::PipeCapture( int fd, Glib::ustring &string ) : buff( string ),
                                                             linestart( 0 ), cursor( 0 ), lineend( 0 )
 {
+	buff.clear();
 	// tie fd to string
 	// make channel
 	channel = Glib::IOChannel::create_from_fd( fd );

src/Utils.cc

@@ -601,8 +601,6 @@ int Utils::execute_command( const Glib::ustring & command,
 	fcntl( out, F_SETFL, O_NONBLOCK );
 	fcntl( err, F_SETFL, O_NONBLOCK );
 	g_child_watch_add( pid, _store_exit_status, &status );
-	output.clear();
-	error.clear();
 	//Lock mutex so we have time to setup pipecapture for output and error streams
 	//  before connecting the input/output signal handler
 	if( !status.foreground )