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

@@ -165,7 +165,8 @@ void av_log(void* avcl, int level, const char *fmt, ...)
 
 void av_vlog(void* avcl, int level, const char *fmt, va_list vl)
 {
-    av_log_callback(avcl, level, fmt, vl);
+    if(av_log_callback)
+        av_log_callback(avcl, level, fmt, vl);
 }
 
 int av_log_get_level(void)