Allow HotStandbyActiveInReplay() to be called in single user mode.

HotStandbyActiveInReplay, introduced in 061b079, only allowed WAL
replay to happen in the startup process, missing the single user case.

This buglet is fairly harmless as it only causes problems when single
user mode in an assertion enabled build is used to replay a btree vacuum
record.

Backpatch to 9.2. 061b079 was backpatched further, but the assertion
was not.

Author: anarazel

src/backend/access/transam/xlog.c

@@ -6442,7 +6442,7 @@ HotStandbyActive(void)
 bool
 HotStandbyActiveInReplay(void)
 {
-	Assert(AmStartupProcess());
+	Assert(AmStartupProcess() || !IsPostmasterEnvironment);
 	return LocalHotStandbyActive;
 }