Fix uninitialized value warning in QFontEngineFT.

When running an application which called e.g. QPainter::drawText under
Valgrind, it would produce "Conditional jump or move depends on
uninitialized value(s)" warnings, since we were allocating a buffer for
a FreeType bitmap without initializing its contents. FreeType,
apparently, does not set the value of all bytes in a bitmap buffer when
it is used as a FT_Bitmap, so we were left with some uninitialized
memory which was still being used.

This commit fixes these warnings, and prevents any potential undefined
behavior.

[ChangeLog][QtGui][General] fixed use of uninitialized memory in the
FreeType font engine

Change-Id: Ia7b3595c78310ce41f76cb4546fc36016c0000a8
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qfontengine_ft.cpp

@@ -1062,6 +1062,7 @@ QFontEngineFT::Glyph *QFontEngineFT::loadGlyph(QGlyphSet *set, uint glyph,
                  (format == Format_A8 ? (info.width + 3) & ~3 : info.width * 4));
     glyph_buffer_size = pitch * info.height;
     glyph_buffer = new uchar[glyph_buffer_size];
+    memset(glyph_buffer, 0, glyph_buffer_size);
 
     if (slot->format == FT_GLYPH_FORMAT_OUTLINE) {
         FT_Bitmap bitmap;