Bug#56120: Failed assertion on MIX/MAX on negative time value

The Item_cache_datetime::val_str function wasn't taking into account that time
could be negative. This led to failed assertion.
Now Item_cache_datetime::val_str correctly converts negative time values
from integer to string representation.

mysql-test/r/func_group.result

@@ -1715,7 +1715,7 @@ drop table t1;
 #
 End of 5.1 tests
 #
-# Bug#55648: Server crash on MIX/MAX on maximum time value
+# Bug#55648: Server crash on MIN/MAX on maximum time value
 #
 CREATE TABLE t1(c1 TIME NOT NULL);
 INSERT INTO t1 VALUES('837:59:59');
@@ -1725,3 +1725,13 @@ MAX(c1)
 838:59:59
 DROP TABLE t1;
 # End of the bug#55648
+#
+# Bug#56120: Failed assertion on MIN/MAX on negative time value
+#
+CREATE TABLE t1(c1 TIME NOT NULL);
+INSERT INTO t1 VALUES('-00:00:01');
+SELECT MAX(c1),MIN(c1) FROM t1;
+MAX(c1)	MIN(c1)
+-00:00:01	-00:00:01
+DROP TABLE t1;
+# End of the bug#56120

mysql-test/t/func_group.test

@@ -1086,7 +1086,7 @@ drop table t1;
 --echo End of 5.1 tests
 
 --echo #
---echo # Bug#55648: Server crash on MIX/MAX on maximum time value
+--echo # Bug#55648: Server crash on MIN/MAX on maximum time value
 --echo #
 CREATE TABLE t1(c1 TIME NOT NULL);
 INSERT INTO t1 VALUES('837:59:59');
@@ -1095,3 +1095,12 @@ SELECT MAX(c1) FROM t1;
 DROP TABLE t1;
 --echo # End of the bug#55648
 
+--echo #
+--echo # Bug#56120: Failed assertion on MIN/MAX on negative time value
+--echo #
+CREATE TABLE t1(c1 TIME NOT NULL);
+INSERT INTO t1 VALUES('-00:00:01');
+SELECT MAX(c1),MIN(c1) FROM t1;
+DROP TABLE t1;
+--echo # End of the bug#56120
+

sql/item.cc

@@ -7510,9 +7510,14 @@ String *Item_cache_datetime::val_str(String *str)
         return NULL;
       if (cached_field_type == MYSQL_TYPE_TIME)
       {
-        ulonglong time= int_value;
-        DBUG_ASSERT(time <= TIME_MAX_VALUE);
+        longlong time= int_value;
         set_zero_time(&ltime, MYSQL_TIMESTAMP_TIME);
+        if (time < 0)
+        {
+          time= -time;
+          ltime.neg= TRUE;
+        }
+        DBUG_ASSERT(time <= TIME_MAX_VALUE);
         ltime.second= time % 100;
         time/= 100;
         ltime.minute= time % 100;