Silence -Wmaybe-uninitialized compiler warnings in dbcommands.c.

When compiling postgres using gcc -O3, there are false-positive
warnings about the now initialized variables. Silence them.

Author: Peter Eisentraut, Andres Freund
Discussion: https://postgr.es/m/15fb2350-b8b8-e188-278f-0b34fdee5210@2ndquadrant.com

src/backend/commands/dbcommands.c

@@ -26,6 +26,7 @@
 #include "access/genam.h"
 #include "access/heapam.h"
 #include "access/htup_details.h"
+#include "access/multixact.h"
 #include "access/tableam.h"
 #include "access/xact.h"
 #include "access/xloginsert.h"
@@ -103,14 +104,14 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	Relation	rel;
 	Oid			src_dboid;
 	Oid			src_owner;
-	int			src_encoding;
-	char	   *src_collate;
-	char	   *src_ctype;
+	int			src_encoding = -1;
+	char	   *src_collate = NULL;
+	char	   *src_ctype = NULL;
 	bool		src_istemplate;
 	bool		src_allowconn;
-	Oid			src_lastsysoid;
-	TransactionId src_frozenxid;
-	MultiXactId src_minmxid;
+	Oid			src_lastsysoid = InvalidOid;
+	TransactionId src_frozenxid = InvalidTransactionId;
+	MultiXactId src_minmxid = InvalidMultiXactId;
 	Oid			src_deftablespace;
 	volatile Oid dst_deftablespace;
 	Relation	pg_database_rel;