Fix segmentation fault that an empty prepared statement could cause.

Back-patch to all supported branches.

Per bug #11335 from Haruka Takatsuka

src/backend/tcop/utility.c

@@ -2308,6 +2308,9 @@ GetCommandLogLevel(Node *parsetree)
 {
 	LogStmtLevel lev;
 
+	if (parsetree == NULL)
+		return LOGSTMT_ALL;
+
 	switch (nodeTag(parsetree))
 	{
 			/* raw plannable queries */