msmpeg4v1: fix undefined behaviour in msmpeg4_decode_picture_header()

Because the order of evaluation of subexpressions is undefined, two
get_bits() calls may not be part of the same expression.  In this
specific case, using get_bits_long() is simpler.

This fixes msmpeg4v1 decoding with armcc.

Originally committed as revision 24902 to svn://svn.ffmpeg.org/ffmpeg/trunk
