cmdutils: fix success path

Since commit 934f2d2f,
cmdutils_read_file() prints a confusing message on success:

IO error: Success

This is because the error message is printed on the success path as
well. Add the missing condition so that it is only printed on error.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

cmdutils.c

@@ -1912,7 +1912,8 @@ int cmdutils_read_file(const char *filename, char **bufptr, size_t *size)
     }
 
 out:
-    av_log(NULL, AV_LOG_ERROR, "IO error: %s\n", av_err2str(ret));
+    if (ret < 0)
+        av_log(NULL, AV_LOG_ERROR, "IO error: %s\n", av_err2str(ret));
     fclose(f);
     return ret;
 }