[sfnt] Fix Savannah bug #43680.

This adds an additional constraint to make the fix from 2013-01-25
really work.

* src/sfnt/ttsbit.c (tt_sbit_decoder_load_image) <index_format==4>:
Check `p' before `num_glyphs'.

ChangeLog

@@ -1,3 +1,13 @@
+2014-11-24  Werner Lemberg  <wl@gnu.org>
+
+	[sfnt] Fix Savannah bug #43680.
+
+	This adds an additional constraint to make the fix from 2013-01-25
+	really work.
+
+	* src/sfnt/ttsbit.c (tt_sbit_decoder_load_image) <index_format==4>:
+	Check `p' before `num_glyphs'.
+
 2014-11-24  Werner Lemberg  <wl@gnu.org>
 
 	[truetype] Fix Savannah bug #43679.

src/sfnt/ttsbit.c

@@ -1170,7 +1170,8 @@
         num_glyphs = FT_NEXT_ULONG( p );
 
         /* overflow check for p + ( num_glyphs + 1 ) * 4 */
-        if ( num_glyphs > (FT_ULong)( ( ( p_limit - p ) >> 2 ) - 1 ) )
+        if ( p + 4 > p_limit                                         ||
+             num_glyphs > (FT_ULong)( ( ( p_limit - p ) >> 2 ) - 1 ) )
           goto NoBitmap;
 
         for ( mm = 0; mm < num_glyphs; mm++ )