MDEV-37033 UBSAN: row_log_table_apply_ops runtime error: applying non-zero offset 1048576 to null pointer

In a UBSAN debug build, the comparisons with next_mrec_end are made
with index->online_log's head/tail members' block ptr with a sort buffer
size offset (1048576).

The logic that flows though to this point means that even srv_sort_buf_size
above a null pointer wouldn't contain the value of next_mrec_end.

As such this is a UBSAN type fix where we first check if the
head.block / tail.block is null before doing the asserts around
this debug condition. This would be required for the assertions
conditions not to segfault anyway.

Author: grooverdan

storage/innobase/row/row0log.cc

@@ -2691,7 +2691,8 @@ row_log_table_apply_ops(
 	ut_ad((mrec == NULL) == (index->online_log->head.bytes == 0));
 
 #ifdef UNIV_DEBUG
-	if (next_mrec_end == index->online_log->head.block
+	if (index->online_log->head.block &&
+	    next_mrec_end == index->online_log->head.block
 	    + srv_sort_buf_size) {
 		/* If tail.bytes == 0, next_mrec_end can also be at
 		the end of tail.block. */
@@ -2706,7 +2707,8 @@ row_log_table_apply_ops(
 			ut_ad(index->online_log->tail.blocks
 			      > index->online_log->head.blocks);
 		}
-	} else if (next_mrec_end == index->online_log->tail.block
+	} else if (index->online_log->tail.block &&
+		   next_mrec_end == index->online_log->tail.block
 		   + index->online_log->tail.bytes) {
 		ut_ad(next_mrec == index->online_log->tail.block
 		      + index->online_log->head.bytes);