env: Check unsetenv() return value

./env -u "" foo and ./env -u "A=123" foo should both fail.
diff --git a/env.c b/env.c
index 4da2630..5d7e8a5 100644
--- a/env.c
+++ b/env.c
@@ -25,7 +25,8 @@
 		*environ = NULL;
 		break;
 	case 'u':
-		unsetenv(EARGF(usage()));
+		if (unsetenv(EARGF(usage())) < 0)
+			eprintf("unsetenv:");
 		break;
 	default:
 		usage();