Avoid invalid memory read/crash when frame offset >= 0xfffffff8.
Base64-encoded example: AAABADAwMDAwMAAAMAAwMDAw/P///w==
(The previous commit verifies that p->buf_size >= 22.)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 56e2cd9c)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>