ffprobe: prefer av_freep() over av_free() for global print_format option.

It could be re-used for instance in case ffprobe would support multiple
input files.

ffprobe.c

@@ -842,7 +842,7 @@ static int probe_file(const char *filename)
     writer_close(&wctx);
 
 end:
-    av_free(print_format);
+    av_freep(&print_format);
 
     return ret;
 }