Correct statement to actually be the intended assert statement.

e3f4cfc7 introduced a LWLockHeldByMe() call, without the corresponding
Assert() surrounding it.

Spotted by Coverity.

Backpatch: 9.1+, like the previous commit

src/backend/storage/buffer/bufmgr.c

@@ -3004,7 +3004,7 @@ FlushOneBuffer(Buffer buffer)
 
 	bufHdr = GetBufferDescriptor(buffer - 1);
 
-	LWLockHeldByMe(bufHdr->content_lock);
+	Assert(LWLockHeldByMe(bufHdr->content_lock));
 
 	FlushBuffer(bufHdr, NULL);
 }