fixup! mingw: spawned processes need to inherit only standard handles

When we suppress the warning that we could not restrict file handle
inheritance because of known error conditions, we should really suppress
the *entire* warning, not just parts of it.

This was reported in
#1578 (comment)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1793,7 +1793,7 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 		ret = CreateProcessW(*wcmd ? wcmd : NULL, wargs, NULL, NULL,
 				     TRUE, flags, wenvblk, dir ? wdir : NULL,
 				     &si.StartupInfo, &pi);
-		if (ret) {
+		if (ret && buf.len) {
 			errno = err_win_to_posix(GetLastError());
 			warning("failed to restrict file handles (%ld)\n\n%s",
 				err, buf.buf);