Add assertion to enforce check of an implicit invariant and add comment about

retry of autoinc read semantics. We always reread the table's autoinc counter
after attempting to initialize it i.e., we want to guarantee that a read of
autoinc valus that is returned to the caller is always covered by the
AUTOINC locking mechanism.

handler/ha_innodb.cc

@@ -7230,6 +7230,11 @@ ha_innobase::innobase_get_auto_increment(
 {
 	ulint		error;
 
+	ut_a(*value == 0);
+
+	/* Note: If the table is not initialized when we attempt the
+	read below. We initialize the table's auto-inc counter  and
+	always do a reread of the AUTOINC value. */
 	do {
 		error = innobase_autoinc_lock();