[sfnt] Pointer sanity checks before reading layer info in 'COLR' v0

* src/sfnt/ttcolr.c (tt_face_get_colr_layer): Check that the pointer to
read from is within the 'COLR' table.

Reported as

  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=50633

src/sfnt/ttcolr.c

@@ -481,7 +481,9 @@
       iterator->p = colr->layers + offset;
     }
 
-    if ( iterator->layer >= iterator->num_layers )
+    if ( iterator->layer >= iterator->num_layers                     ||
+         iterator->p < colr->layers                                  ||
+         iterator->p >= ( (FT_Byte*)colr->table + colr->table_size ) )
       return 0;
 
     *aglyph_index = FT_NEXT_USHORT( iterator->p );