install(1): addition to r363067

Also, make it not break if STRIPBIN points to strip version without -o support
and destination does not exist before installing.

Reported by:	lwhsu
MFC after:      1 month
X-MFC-With:     363064

usr.bin/xinstall/xinstall.c

@@ -1350,9 +1350,9 @@ strip(const char *to_name, const char *from_name, char **dresp)
 		/* NOTREACHED */
 	}
 	if (status != 0) {
-		(void)unlink(to_name);
 		if (from_name != NULL)
 			return (0);
+		(void)unlink(to_name);
 		errx(EX_SOFTWARE, "strip command %s failed on %s",
 		    stripbin, to_name);
 	}