Suppress uninitialized-variable warning from a61b1f748.

Some compilers complain about sub_rteperminfos not being
initialized, evidently because they don't detect that it
is only used and set if isGeneralSelect is true.
Make it follow the long-established pattern for its
sibling variable sub_rtable.

Per reports from Pavel Stehule and the buildfarm.

Discussion: https://postgr.es/m/CAFj8pRDOvGOi-n616kM0Cc7qSbg_nGoS=-haB+D785sUXADqSg@mail.gmail.com

src/backend/parser/analyze.c

@@ -615,6 +615,7 @@ transformInsertStmt(ParseState *pstate, InsertStmt *stmt)
 	else
 	{
 		sub_rtable = NIL;		/* not used, but keep compiler quiet */
+		sub_rteperminfos = NIL;
 		sub_namespace = NIL;
 	}