MDEV-7679: ANALYZE crashes when printing WHERE when no default db

Fix Item_ident::print() to work when there is no current database

mysql-test/r/analyze_format_json.result

@@ -210,3 +210,32 @@ EXPLAIN
   }
 }
 drop table t1,t2;
+create table t1(a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+#
+# MDEV-7679: ANALYZE crashes when printing WHERE when no default db
+#
+select database();
+database()
+test
+select database();
+database()
+NULL
+analyze format=json select * from test.t1 where t1.a<5;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t1",
+      "access_type": "ALL",
+      "r_loops": 1,
+      "rows": 10,
+      "r_rows": 10,
+      "filtered": 100,
+      "r_filtered": 50,
+      "attached_condition": "(test.t1.a < 5)"
+    }
+  }
+}
+drop table t1;

mysql-test/t/analyze_format_json.test

@@ -51,3 +51,19 @@ analyze format=json select * from t1 straight_join t2 force index(a) where t2.a=
 
 drop table t1,t2;
 
+create table t1(a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+--echo #
+--echo # MDEV-7679: ANALYZE crashes when printing WHERE when no default db
+--echo #
+
+select database();
+connect (con1,localhost,root,,*NO-ONE*);
+connection con1;
+select database();
+analyze format=json select * from test.t1 where t1.a<5;
+disconnect con1;
+connection default;
+drop table t1;
+

sql/item.cc

@@ -2380,7 +2380,8 @@ void Item_ident::print(String *str, enum_query_type query_type)
       When printing EXPLAIN, don't print database name when it's the same as
       current database.
     */
-    bool skip_db= (query_type & QT_EXPLAIN) && !strcmp(thd->db, db_name);
+    bool skip_db= (query_type & QT_EXPLAIN) && thd->db && 
+                  !strcmp(thd->db, db_name);
     if (!skip_db && 
         !(cached_table && cached_table->belong_to_view &&
           cached_table->belong_to_view->compact_view_format))