null processing for character_set_results (BUG#3296)

mysql-test/r/query_cache.result

@@ -846,4 +846,9 @@ SELECT * FROM test.t1;
 a
 USE test;
 DROP TABLE t1;
+set character_set_results=null;
+select @@character_set_results;
+@@character_set_results
+NULL
+set character_set_results=default;
 SET GLOBAL query_cache_size=0;

mysql-test/t/query_cache.test

@@ -620,5 +620,11 @@ SELECT * FROM test.t1;
 USE test;
 DROP TABLE t1;
 
+#
+# charset with NULL
+#
+set character_set_results=null;
+select @@character_set_results;
+set character_set_results=default;
 
 SET GLOBAL query_cache_size=0;

sql/sql_cache.cc

@@ -780,7 +780,9 @@ void Query_cache::store_query(THD *thd, TABLE_LIST *tables_used)
     flags.character_set_client_num=
       thd->variables.character_set_client->number;
     flags.character_set_results_num=
-      thd->variables.character_set_results->number;
+      (thd->variables.character_set_results ?
+       thd->variables.character_set_results->number :
+       UINT_MAX);
     flags.collation_connection_num=
       thd->variables.collation_connection->number;
     flags.limit= thd->variables.select_limit;
@@ -965,7 +967,9 @@ Query_cache::send_result_to_client(THD *thd, char *sql, uint query_length)
 			   1 : 0);
   flags.character_set_client_num= thd->variables.character_set_client->number;
   flags.character_set_results_num=
-    thd->variables.character_set_results->number;
+    (thd->variables.character_set_results ?
+     thd->variables.character_set_results->number :
+     UINT_MAX);
   flags.collation_connection_num= thd->variables.collation_connection->number;
   flags.limit= thd->variables.select_limit;
   memcpy((void *)(sql + (tot_length - QUERY_CACHE_FLAGS_SIZE)),