Clear config item writing buffer before freeing it

This buffer may be used to store items like passwords, so better clean
it explicitly to avoid possibility of leaving such items in heap memory
unnecessarily.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

wpa_supplicant/config_file.c

@@ -493,7 +493,7 @@ static void write_str(FILE *f, const char *field, struct wpa_ssid *ssid)
 	if (value == NULL)
 		return;
 	fprintf(f, "\t%s=%s\n", field, value);
-	os_free(value);
+	str_clear_free(value);
 }