Fix nbtree lookahead overflow bug.

Add bounds checking to nbtree's lookahead/skip-within-a-page mechanism.
Otherwise it's possible for cases with lots of before-array-keys tuples
to overflow an int16 variable, causing the mechanism to generate an out
of bounds page offset number.

Oversight in commit 5bf748b, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/6c68ac42-bbb5-8b24-103e-af0e279c536f@gmail.com
Backpatch: 17-, where nbtree SAOP execution was enhanced.

Author: petergeoghegan

src/backend/access/nbtree/nbtutils.c

@@ -4091,7 +4091,7 @@ _bt_checkkeys_look_ahead(IndexScanDesc scan, BTReadPageState *pstate,
 	 */
 	if (!pstate->targetdistance)
 		pstate->targetdistance = LOOK_AHEAD_DEFAULT_DISTANCE;
-	else
+	else if (pstate->targetdistance < MaxIndexTuplesPerPage / 2)
 		pstate->targetdistance *= 2;
 
 	/* Don't read past the end (or before the start) of the page, though */