MDEV-16984 Assertion `dec' failed in Dec_ptr::cmp

Fixing the bug (in the code from MDEV-16910)

mysql-test/main/type_decimal.result

@@ -1204,5 +1204,19 @@ Warnings:
 Note	1003	select `test`.`t1`.`a` AS `a` from `test`.`t1` where <cache>(octet_length(10.0)) + `test`.`t1`.`a` <=> <cache>(octet_length(10.00)) + `test`.`t1`.`a`
 DROP TABLE t1;
 #
+# MDEV-16984 Assertion `dec' failed in Dec_ptr::cmp
+#
+SET sql_mode='';
+CREATE TABLE t1 (dc decimal(10));
+INSERT INTO t1 VALUES (0000000),(NULL);
+SELECT 1 FROM t1 GROUP BY 'm' <=> dc;
+1
+1
+1
+Warnings:
+Warning	1292	Truncated incorrect DECIMAL value: 'm'
+DROP TABLE t1;
+SET sql_mode=DEFAULT;
+#
 # End of 10.4 tests
 #

mysql-test/main/type_decimal.test

@@ -744,6 +744,17 @@ EXECUTE IMMEDIATE 'EXPLAIN EXTENDED SELECT * FROM t1 WHERE LENGTH(10.0)+a<=>LENG
 DROP TABLE t1;
 
 
+--echo #
+--echo # MDEV-16984 Assertion `dec' failed in Dec_ptr::cmp
+--echo #
+SET sql_mode='';
+CREATE TABLE t1 (dc decimal(10));
+INSERT INTO t1 VALUES (0000000),(NULL);
+SELECT 1 FROM t1 GROUP BY 'm' <=> dc;
+DROP TABLE t1;
+SET sql_mode=DEFAULT;
+
+
 --echo #
 --echo # End of 10.4 tests
 --echo #

sql/item_cmpfunc.cc

@@ -823,7 +823,7 @@ int Arg_comparator::compare_e_real()
 int Arg_comparator::compare_e_decimal()
 {
   VDec val1(*a), val2(*b);
-  if (val1.is_null() || val1.is_null())
+  if (val1.is_null() || val2.is_null())
     return MY_TEST(val1.is_null() && val2.is_null());
   return MY_TEST(val1.cmp(val2) == 0);
 }