[sfnt] Guard access in 'COLR' v1 glyph binary search.

Reported as

  https://bugs.chromium.org/p/chromium/issues/detail?id=1505216

* src/sfnt/ttcolr.c (find_base_glyph_v1_record): Guard access of the search
pointer during binary search.  The pointer needs to be checked as we go as
the test that compares number of v1 glyphs with table size at the time of
loading the table is not sufficient on its own.

A scenario is possible in which the `BaseGlyphRecord` list extends into
non-`BaseGlyphRecord` parts of the 'COLR' v1 table (but passed the size
comparison check).  Then, at those locations, invalid glyph ID values are
read and may provoke an invalid read due to reassigning min and max values
during the binary search.

src/sfnt/ttcolr.c

@@ -1269,6 +1269,7 @@
   static FT_Bool
   find_base_glyph_v1_record( FT_Byte *           base_glyph_begin,
                              FT_UInt             num_base_glyph,
+                             FT_Byte *           end_colr,
                              FT_UInt             glyph_id,
                              BaseGlyphV1Record  *record )
   {
@@ -1287,6 +1288,14 @@
        */
       FT_Byte  *p = base_glyph_begin + 4 + mid * BASE_GLYPH_PAINT_RECORD_SIZE;
 
+
+      /* We need to be able to read 2 bytes (FT_NEXT_USHORT) for the glyph */
+      /* ID, then 4 bytes (FT_NEXT_ULONG) for the paint offset.  If that's */
+      /* not available before the end of the table, something's wrong with */
+      /* the font and we can't find a COLRv1 glyph.                        */
+      if ( p > end_colr - 2 - 4 )
+        return 0;
+
       FT_UShort  gid = FT_NEXT_USHORT( p );
 
 
@@ -1328,6 +1337,7 @@
 
     if ( !find_base_glyph_v1_record( colr->base_glyphs_v1,
                                      colr->num_base_glyphs_v1,
+                                     (FT_Byte*)colr->table + colr->table_size,
                                      base_glyph,
                                      &base_glyph_v1_record ) )
       return 0;