Fix variable initialization with ALTER SUBSCRIPTION DROP PUBLICATION

copy_data is not a supported option with this sub-command of ALTER
SUBSCRIPTION, which would not make a variable related to it initialized
after parsing the option set in DefElems.  A refresh could then refer to
it.

Author: Ranier Vilela
Reviewed-by: Peter Smith
Discussion: https://postgr.es/m/CAEudQAp5P8nr=ze2Gv=BMj=DJFZnrvendZCZcC-fos3QiDe2sg@mail.gmail.com

src/backend/commands/subscriptioncmds.c

@@ -949,7 +949,7 @@ AlterSubscription(AlterSubscriptionStmt *stmt, bool isTopLevel)
 		case ALTER_SUBSCRIPTION_DROP_PUBLICATION:
 			{
 				bool		isadd = stmt->kind == ALTER_SUBSCRIPTION_ADD_PUBLICATION;
-				bool		copy_data;
+				bool		copy_data = false;
 				bool		refresh;
 				List	   *publist;