avcodec/mpegvideo_enc: Clip bits_per_raw_sample within valid range

Fixes out of array read
Fixes: test_case-mdc.264 (b47be15a120979f5a1a945c938cbef33)

Found-by: Tyson Smith <twsmith@mozilla.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
