Add missing check for malloc failure in plpgsql_extra_checks_check_hook().

Per report from Andreas Seltenreich.  Back-patch to affected versions.

Report: <874m8nn0hv.fsf@elite.ansel.ydns.eu>

src/pl/plpgsql/src/pl_handler.c

@@ -108,6 +108,8 @@ plpgsql_extra_checks_check_hook(char **newvalue, void **extra, GucSource source)
 	}
 
 	myextra = (int *) malloc(sizeof(int));
+	if (!myextra)
+		return false;
 	*myextra = extrachecks;
 	*extra = (void *) myextra;