Add a notice telling that the behavior of skip_put_bits() is undefined

if n is 0.

Originally committed as revision 18285 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/bitstream.h

@@ -337,6 +337,7 @@ static inline void skip_put_bytes(PutBitContext *s, int n){
 /**
  * Skips the given number of bits.
  * Must only be used if the actual values in the bitstream do not matter.
+ * If \p n is 0 the behavior is undefined.
  */
 static inline void skip_put_bits(PutBitContext *s, int n){
 #ifdef ALT_BITSTREAM_WRITER