[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
1 file changed