evas: sw font draw - protect against null pointer access

The image data of dst could be null in a rare case.
@fix

Reviewed-by: Christopher Michael <devilhorns@comcast.net>
Differential Revision: https://phab.enlightenment.org/D12163

src/lib/evas/common/evas_font_draw.c

@@ -457,6 +457,8 @@ evas_common_font_glyph_draw(RGBA_Font_Glyph *fg,
    DATA32 coltab[16], col;
    DATA16 mtab[16], v;
 
+   if (!dst) return;
+
    // FIXME: Use dw, dh for scaling glyphs...
    (void) dw;
    (void) dh;