lavf: fix memleak

Fixes CID747738
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -432,6 +432,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
         if (!av_strcasecmp(mime_type, "audio/aacp")) {
             *fmt = av_find_input_format("aac");
         }
+        av_freep(&mime_type);
     }
 
     for(probe_size= PROBE_BUF_MIN; probe_size<=max_probe_size && !*fmt;