lp:923429 Crash in decimal_cmp on using UNIX_TIMESTAMP with a wrongly formatted timestamp

UNIX_TIMESTAMP() can be null, and returns null for invalid values

mysql-test/r/type_timestamp.result

@@ -587,3 +587,16 @@ Variable_name	Value
 Handler_read_next	1
 DROP TABLE t1, t2;
 End of 5.1 tests
+#
+# lp:923429 Crash in decimal_cmp on using UNIX_TIMESTAMP with a wrongly formatted timestamp
+#
+SELECT UNIX_TIMESTAMP('abc') > 0;
+UNIX_TIMESTAMP('abc') > 0
+NULL
+Warnings:
+Warning	1292	Incorrect datetime value: 'abc'
+SELECT UNIX_TIMESTAMP('abc');
+UNIX_TIMESTAMP('abc')
+NULL
+Warnings:
+Warning	1292	Incorrect datetime value: 'abc'

mysql-test/t/type_timestamp.test

@@ -410,3 +410,10 @@ SHOW STATUS LIKE 'Handler_read_next';
 DROP TABLE t1, t2;
 
 --echo End of 5.1 tests
+
+--echo #
+--echo # lp:923429 Crash in decimal_cmp on using UNIX_TIMESTAMP with a wrongly formatted timestamp
+--echo #
+SELECT UNIX_TIMESTAMP('abc') > 0;
+SELECT UNIX_TIMESTAMP('abc');
+

sql/item_timefunc.cc

@@ -1134,15 +1134,7 @@ bool Item_func_unix_timestamp::get_timestamp_value(my_time_t *seconds,
 
   MYSQL_TIME ltime;
   if (get_arg0_date(&ltime, 0))
-  {
-    /*
-      We have to set null_value again because get_arg0_date will also set it
-      to true if we have wrong datetime parameter (and we should return 0 in 
-      this case).
-    */
-    null_value= args[0]->null_value;
     return 1;
-  }
 
   uint error_code;
   *seconds= TIME_to_timestamp(current_thd, &ltime, &error_code);

sql/item_timefunc.h

@@ -385,11 +385,6 @@ class Item_func_unix_timestamp :public Item_func_seconds_hybrid
   Item_func_unix_timestamp(Item *a) :Item_func_seconds_hybrid(a) {}
   const char *func_name() const { return "unix_timestamp"; }
   bool check_partition_func_processor(uchar *int_arg) {return FALSE;}
-  void fix_num_length_and_dec()
-  {
-    maybe_null= false;
-    Item_func_seconds_hybrid::fix_num_length_and_dec();
-  }
   /*
     UNIX_TIMESTAMP() depends on the current timezone
     (and thus may not be used as a partitioning function)