Revert get_num() to old behaviour in order to handle

incorrect RMs generated by lavf muxer.

Originally committed as revision 10892 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rmdec.c

@@ -366,7 +366,7 @@ static int get_num(ByteIOContext *pb, int *len)
 
     n = get_be16(pb);
     (*len)-=2;
-    n &= 0x7FFF;
+//    n &= 0x7FFF;
     if (n >= 0x4000) {
         return n - 0x4000;
     } else {