avutil/log: allow av_log_set_callback (NULL)

Idea-by: Don Moir <donmoir@comcast.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/log.c

@@ -165,6 +165,7 @@ void av_log(void* avcl, int level, const char *fmt, ...)
 
 void av_vlog(void* avcl, int level, const char *fmt, va_list vl)
 {
+    if(av_log_callback)
         av_log_callback(avcl, level, fmt, vl);
 }