Fix up panics when fork fails due to hitting proc limit

The function clearning credentials on failure asserts the process is a
zombie, which is not true when fork fails.

Changing creds to NULL is unnecessary, but is still being done for
consistency with other code.

Pointy hat: mjg
Reported by: pho

sys/kern/kern_fork.c

@@ -943,7 +943,8 @@ fail:
 #endif
 	racct_proc_exit(newproc);
 fail1:
-	crfree(proc_set_cred(newproc, NULL));
+	crfree(newproc->p_ucred);
+	newproc->p_ucred = NULL;
 fail2:
 	if (vm2 != NULL)
 		vmspace_free(vm2);