Fix text disappearing after space (bug #6617)

What happened here is that when fixing bug #5734 we cahnged the
glyph image cache lookup to never return a NULL pointer, but to
instead return a 0x0 image surface. We didn't also update code
that was looking for a NULL pointer here, and as such we were
calling XRenderAddGlyphs with a 0x0 image which apparently
tickles a bug in some X servers.

src/cairo-xlib-surface.c

@@ -2063,7 +2063,7 @@ _xlib_glyphset_cache_create_entry (void *abstract_cache,
     entry->key = *key;
     _cairo_unscaled_font_reference (entry->key.unscaled);
 
-    if (!im->image) {
+    if (im->image->width == 0 || im->image->height == 0) {
 	entry->glyph = None;
 	entry->glyphset = None;
 	entry->key.base.memory = 0;