Fix targetRelation initializiation in prepsecurity

In 6f9bd50e, we modified
expand_security_quals() to tell expand_security_qual() about when the
current RTE was the targetRelation.  Unfortunately, that commit
initialized the targetRelation variable used outside of the loop over
the RTEs instead of at the start of it.

This patch moves the variable and the initialization of it into the
loop, where it should have been to begin with.

Pointed out by Dean Rasheed.

Back-patch to 9.4 as the original commit was.

src/backend/optimizer/prep/prepsecurity.c

@@ -63,7 +63,6 @@ expand_security_quals(PlannerInfo *root, List *tlist)
 	Query	   *parse = root->parse;
 	int			rt_index;
 	ListCell   *cell;
-	bool		targetRelation = false;
 
 	/*
 	 * Process each RTE in the rtable list.
@@ -74,6 +73,7 @@ expand_security_quals(PlannerInfo *root, List *tlist)
 	rt_index = 0;
 	foreach(cell, parse->rtable)
 	{
+		bool			targetRelation = false;
 		RangeTblEntry  *rte = (RangeTblEntry *) lfirst(cell);
 
 		rt_index++;