MDEV-15580: Assertion `!lex->explain' failed in lex_start(THD*):

Problem was that we did not delete explain information
when Galera must replay a query.

Could not find easily repeatable test case that would not
cause other problems.

sql/sql_parse.cc

@@ -7741,6 +7741,8 @@ static void wsrep_mysql_parse(THD *thd, char *rawbuf, uint length,
 
       if (thd->wsrep_conflict_state == MUST_REPLAY)
       {
+	if (thd->lex->explain)
+          delete_explain_query(thd->lex);
         wsrep_replay_transaction(thd);
       }
 
@@ -7793,8 +7795,12 @@ static void wsrep_mysql_parse(THD *thd, char *rawbuf, uint length,
     }
 
     /* If retry is requested clean up explain structure */
-    if (thd->wsrep_conflict_state == RETRY_AUTOCOMMIT && thd->lex->explain)
+    if ((thd->wsrep_conflict_state == RETRY_AUTOCOMMIT ||
+	 thd->wsrep_conflict_state == MUST_REPLAY )
+	&& thd->lex->explain)
+    {
         delete_explain_query(thd->lex);
+    }
 
   }  while (thd->wsrep_conflict_state== RETRY_AUTOCOMMIT);