h264: prevent an out of array read in decode_nal_units()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -3646,7 +3646,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             s->workaround_bugs |= FF_BUG_TRUNCATED;
 
         if(!(s->workaround_bugs & FF_BUG_TRUNCATED)){
-        while(ptr[dst_length - 1] == 0 && dst_length > 0)
+        while(dst_length > 0 && ptr[dst_length - 1] == 0)
             dst_length--;
         }
         bit_length= !dst_length ? 0 : (8*dst_length - ff_h264_decode_rbsp_trailing(h, ptr + dst_length - 1));