av_tempfile: fix leak in error case

Fixes CID733796 Part2
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/file.c

@@ -175,6 +175,7 @@ int av_tempfile(const char *prefix, char **filename, int log_offset, void *log_c
     if (fd < 0) {
         int err = AVERROR(errno);
         av_log(&file_log_ctx, AV_LOG_ERROR, "ff_tempfile: Cannot open temporary file %s\n", *filename);
+        av_freep(filename);
         return err;
     }
     return fd; /* success */