test-prio-queue: use xmalloc

test-prio-queue.c doesn't check the return value of malloc, and could
segfault.

It's unlikely for this to matter in practice; it's a small allocation,
and this code isn't even installed alongside the rest of Git. But let's
use xmalloc(), which makes auditing for other accidental uses of bare
malloc() easier.

Reported-by: 王健强 <jianqiang.wang@securitygossip.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-prio-queue.c

@@ -40,7 +40,7 @@ int cmd__prio_queue(int argc, const char **argv)
 		} else if (!strcmp(*argv, "stack")) {
 			pq.compare = NULL;
 		} else {
-			int *v = malloc(sizeof(*v));
+			int *v = xmalloc(sizeof(*v));
 			*v = atoi(*argv);
 			prio_queue_put(&pq, v);
 		}