When dst_length == 0 bit_length has to be 0, too, but the current code still

When dst_length == 0 bit_length has to be 0, too, but the current code still
calls decode_rbsp_trailing() and therefore bit_length might get negative.
Although the remaining code is able to handle a negative bit_length, avoid
the calculation at all by setting bit_length to 0 for dst_length == 0.
patch by Reinhard Nissl, rnissl gmx de

Originally committed as revision 8690 to svn://svn.ffmpeg.org/ffmpeg/trunk