PG-2014 Initalize nested query stack to fix crash on DDL

The pgsm_ProcessUtility() which handles DDL increments nesting_level but
does not put a query text on the next_queries stack while
pgsm_ExecutorRun() does both. It is unclear to me if this is a mistake
or by design but since readers of the query check for is the query text
pointer is NULL and pgsm_ExecutorRun() reset the pointer to NULL before
returning it is safe as long as we initialize the stack to all NULL
pointers, which we did not.

This bug was found by our test suite in Jenkins on some RHEL based
distro version and seems to mostly happen when the first query of a
backend is CREATE EXTENSION and we have enabled query normalization but
it is entirely possible that it could happen under other circumstances
too.

The use of calloc() over palloc0() is to keep the patch small since the
previous code used malloc().

pg_stat_monitor.c

@@ -343,7 +343,7 @@ _PG_init(void)
 	ExecutorCheckPerms_hook = HOOK(pgsm_ExecutorCheckPerms);
 
 	nested_queryids = (int64 *) malloc(sizeof(int64) * max_stack_depth);
-	nested_query_txts = (char **) malloc(sizeof(char *) * max_stack_depth);
+	nested_query_txts = (char **) calloc(max_stack_depth, sizeof(char *));
 
 	system_init = true;
 }