Document what av_get_token() returns in case of allocation failure.

Originally committed as revision 23596 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/parseutils.h

@@ -38,7 +38,7 @@
  * terminating char
  * @param term a 0-terminated list of terminating chars
  * @return the malloced unescaped string, which must be av_freed by
- * the user
+ * the user, NULL in case of allocation failure
  */
 char *av_get_token(const char **buf, const char *term);