After-merge cleanup for MDEV-27207 + MDEV-31719

Something went wrong during a merge (from 10.5 to 10.6)
of 68403eed
(fixing bugs MDEV-27207 and MDEV-31719).

Originally (in 10.5) the fix was done in_inet6::set() in
plugin/type_inet/sql_type_inet.cc.
In 10.6 this code resides in a different place:
in the method in_fbt::set() of a template class
in sql/sql_type_fixedbin.h.

During the merge:
- the fix did not properly migrate to in_fbt::set()
- the related MTR tests disappeared

This patch fixes in_fbt::set() properly and restores MTR tests.

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

@@ -2257,3 +2257,29 @@ a	m
 ::10	1
 DROP VIEW v1;
 DROP TABLE t1, t2;
+#
+# MDEV-27207 Assertion `!m_null_value' failed in int FixedBinTypeBundle<FbtImpl>::cmp_item_fbt::compare or in cmp_item_inet6::compare
+#
+CREATE TABLE t1 (a CHAR,b INET6);
+SELECT * FROM t1 WHERE (a,b) IN (('',''),('',''));
+a	b
+Warnings:
+Warning	1292	Incorrect inet6 value: ''
+Warning	1292	Incorrect inet6 value: ''
+INSERT INTO t1 VALUES ('','::'),('','::');
+SELECT * FROM t1 WHERE (a,b) IN (('',''),('',''));
+a	b
+Warnings:
+Warning	1292	Incorrect inet6 value: ''
+Warning	1292	Incorrect inet6 value: ''
+DROP TABLE t1;
+#
+# MDEV-31719 Wrong result of: WHERE inet6_column IN ('','::1')
+#
+CREATE OR REPLACE TABLE t1 (a INET6);
+INSERT INTO t1 VALUES ('::');
+SELECT * FROM t1 WHERE a IN ('','::1');
+a
+Warnings:
+Warning	1292	Incorrect inet6 value: ''
+DROP TABLE t1;

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

@@ -1656,3 +1656,22 @@ SELECT * FROM v1 ORDER BY a;
 SELECT * FROM t2 ORDER BY a;
 DROP VIEW v1;
 DROP TABLE t1, t2;
+
+--echo #
+--echo # MDEV-27207 Assertion `!m_null_value' failed in int FixedBinTypeBundle<FbtImpl>::cmp_item_fbt::compare or in cmp_item_inet6::compare
+--echo #
+
+CREATE TABLE t1 (a CHAR,b INET6);
+SELECT * FROM t1 WHERE (a,b) IN (('',''),('',''));
+INSERT INTO t1 VALUES ('','::'),('','::');
+SELECT * FROM t1 WHERE (a,b) IN (('',''),('',''));
+DROP TABLE t1;
+
+--echo #
+--echo # MDEV-31719 Wrong result of: WHERE inet6_column IN ('','::1')
+--echo #
+
+CREATE OR REPLACE TABLE t1 (a INET6);
+INSERT INTO t1 VALUES ('::');
+SELECT * FROM t1 WHERE a IN ('','::1');
+DROP TABLE t1;

sql/sql_type_fixedbin.h

@@ -776,10 +776,12 @@ class Type_handler_fbt: public Type_handler
       Fbt *buff= &((Fbt *) base)[pos];
       Fbt_null value(item);
       if (value.is_null())
+      {
         *buff= Fbt::zero();
-      else
+        return true;
+      }
       *buff= value;
-      return FALSE;
+      return false;
     }
     uchar *get_value(Item *item) override
     {