MDEV-20826 Wrong result of MIN(inet6) with GROUP BY

Type_handler_inet6::is_val_native_ready() was not overriden and
returned "false" by default, so Item_sum_min_max::update_field()
erroneously went through the min_max_update_str_field() rather than
min_max_update_native_field() execution path. As a result '8888::'
was compared to 'fff::' in string format (rather than INET6 binary format)
and gave "less" instead of "greater".

Adding the forgotten overriding method returning "true".

plugin/type_inet/mysql-test/type_inet/type_inet6.result

@@ -1968,3 +1968,12 @@ def					CAST(a AS BINARY(16777215))	250	16777215	0	Y	128	0	63
 def					CAST(a AS BINARY(16777216))	251	16777216	0	Y	128	0	63
 CAST(a AS BINARY(0))	CAST(a AS BINARY(1))	CAST(a AS BINARY(16))	CAST(a AS BINARY(255))	CAST(a AS BINARY(256))	CAST(a AS BINARY(512))	CAST(a AS BINARY(513))	CAST(a AS BINARY(65532))	CAST(a AS BINARY(65533))	CAST(a AS BINARY(65534))	CAST(a AS BINARY(65535))	CAST(a AS BINARY(65536))	CAST(a AS BINARY(16777215))	CAST(a AS BINARY(16777216))
 DROP TABLE t1;
+#
+# MDEV-20826 Wrong result of MIN(inet6) with GROUP BY
+#
+CREATE TABLE t1 (id INT, a INET6) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1, 'fff::'),(1, '8888::');
+SELECT MIN(a), MAX(a) FROM t1 GROUP BY id;
+MIN(a)	MAX(a)
+fff::	8888::
+DROP TABLE t1;

plugin/type_inet/mysql-test/type_inet/type_inet6.test

@@ -1436,3 +1436,12 @@ SELECT
 FROM t1;
 --disable_metadata
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-20826 Wrong result of MIN(inet6) with GROUP BY
+--echo #
+
+CREATE TABLE t1 (id INT, a INET6) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1, 'fff::'),(1, '8888::');
+SELECT MIN(a), MAX(a) FROM t1 GROUP BY id;
+DROP TABLE t1;

plugin/type_inet/sql_type_inet.h

@@ -386,6 +386,7 @@ class Type_handler_inet6: public Type_handler
   }
 
   bool is_scalar_type() const override { return true; }
+  bool is_val_native_ready() const override { return true; }
   bool can_return_int() const override { return false; }
   bool can_return_decimal() const override { return false; }
   bool can_return_real() const override { return false; }