MDEV-5060 Server crashes on EXPLAIN EXTENDED or EXPLAIN PARTITIONS with explain in slow_log

- If we're running explain with flags, use the same set of flags to make EXPLAIN columns 
  and contents.

mysql-test/r/explain_slowquerylog.result

@@ -20,3 +20,9 @@ Error	1146	Table 'test.nonexisting' doesn't exist
 SELECT 1;
 1
 1
+#
+# MDEV-5060 Server crashes on EXPLAIN EXTENDED or EXPLAIN PARTITIONS with explain in slow_log
+#
+EXPLAIN PARTITIONS SELECT 1 ;
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used

mysql-test/t/explain_slowquerylog.test

@@ -28,3 +28,10 @@ SHOW WARNINGS;
 --disconnect con1
 --connection default
 SELECT 1;
+
+--echo #
+--echo # MDEV-5060 Server crashes on EXPLAIN EXTENDED or EXPLAIN PARTITIONS with explain in slow_log
+--echo #
+EXPLAIN PARTITIONS SELECT 1 ;
+
+

sql/opt_qpf.cc

@@ -122,11 +122,13 @@ int QPF_query::print_explain(select_result_sink *output,
   }
 }
 
+
 bool print_qpf_query(LEX *lex, THD *thd, String *str)
 {
   return lex->query_plan_footprint->print_explain_str(thd, str);
 }
 
+
 bool QPF_query::print_explain_str(THD *thd, String *out_str)
 {
   List<Item> fields;
@@ -134,7 +136,7 @@ bool QPF_query::print_explain_str(THD *thd, String *out_str)
 
   select_result_text_buffer output_buf(thd);
   output_buf.send_result_set_metadata(fields, thd->lex->describe);
-  if (print_explain(&output_buf, 0))
+  if (print_explain(&output_buf, thd->lex->describe))
     return true;
   output_buf.save_to(out_str);
   return false;