Mask bits of reference indexes properly to avoid negative numbers entangling

each other.
Fixes at least:
CAMA1_TOSHIBA_B
cama1_vtc_c
CAMA3_Sand_E
cama3_vtc_b
CAMASL3_Sony_B
CVMA1_TOSHIBA_B
CVMAQP3_Sony_D
cvmp_mot_mbaff0_full_B
FRExt/HCAMFF1_HHI
FRExt/HCHP3_HHI_A
FRExt/HVLCMFF0_Sony_B

Originally committed as revision 14683 to svn://svn.ffmpeg.org/ffmpeg/trunk