MDEV-7814 Assertion `args[0]->fixed' fails in Item_func_conv_charset::Item_func_conv_charset

Removing a wrong assertion.

mysql-test/r/ctype_utf8.result

@@ -5134,5 +5134,13 @@ a	1024
 Warnings:
 Warning	1260	Row 2 was cut by GROUP_CONCAT()
 #
+# MDEV-7814 Assertion `args[0]->fixed' fails in Item_func_conv_charset::Item_func_conv_charset
+#
+CREATE TABLE t1(a CHAR(1) CHARACTER SET latin1, b INT NOT NULL);
+CREATE TABLE t2(a CHAR(1) CHARACTER SET utf8 COLLATE utf8_general_ci, b INT NOT NULL);
+SELECT (SELECT t2.a FROM t2 WHERE t2.a=t1.a) AS aa, b, COUNT(b) FROM t1 GROUP BY aa;
+aa	b	COUNT(b)
+DROP TABLE t1,t2;
+#
 # End of 5.5 tests
 #

mysql-test/t/ctype_utf8.test

@@ -1596,6 +1596,14 @@ FROM (SELECT 'a' AS id, REPEAT('foo bar', 100) AS body
 UNION ALL
 SELECT 'a' AS id, REPEAT('bla bla', 100) AS body) t1;
 
+--echo #
+--echo # MDEV-7814 Assertion `args[0]->fixed' fails in Item_func_conv_charset::Item_func_conv_charset
+--echo #
+CREATE TABLE t1(a CHAR(1) CHARACTER SET latin1, b INT NOT NULL);
+CREATE TABLE t2(a CHAR(1) CHARACTER SET utf8 COLLATE utf8_general_ci, b INT NOT NULL);
+SELECT (SELECT t2.a FROM t2 WHERE t2.a=t1.a) AS aa, b, COUNT(b) FROM t1 GROUP BY aa;
+DROP TABLE t1,t2;
+
 --echo #
 --echo # End of 5.5 tests
 --echo #

sql/item_strfunc.h

@@ -827,7 +827,6 @@ class Item_func_conv_charset :public Item_str_func
   Item_func_conv_charset(Item *a, CHARSET_INFO *cs, bool cache_if_const) 
     :Item_str_func(a) 
   {
-    DBUG_ASSERT(args[0]->fixed);
     conv_charset= cs;
     if (cache_if_const && args[0]->const_item() && !args[0]->is_expensive())
     {