Fix potential for compiler warning in GlobalVisTestFor().

In d9d8aa9bb9a I added a defensive NULL assignment to protect against a
not-too-smart compiler warning about unitialized variable use after the
switch. Unfortunately I only did so on master and forgot to adjust that for
14.

Stephen noticed that there actually is a compiler warning :(.
Reported-By: Stephen Frost <sfrost@snowman.net>
Discussion: https://postgr.es/m/20210827224639.GX17906@tamriel.snowman.net

src/backend/storage/ipc/procarray.c

@@ -4027,7 +4027,7 @@ DisplayXidCache(void)
 GlobalVisState *
 GlobalVisTestFor(Relation rel)
 {
-	GlobalVisState *state;
+	GlobalVisState *state = NULL;
 
 	/* XXX: we should assert that a snapshot is pushed or registered */
 	Assert(RecentXmin);