Bug #31440: 'select 1 regex null' asserts debug server

The special case with NULL as a regular expression
was handled at prepare time. But in this special case
the item was not marked as fixed. This caused an assertion
at execution time.
Fixed my marking the item as fixed even when known to 
return NULL at prepare time.

mysql-test/r/func_regexp.result

@@ -98,3 +98,8 @@ R2
 R3
 deallocate prepare stmt1;
 drop table t1;
+End of 4.1 tests
+SELECT 1 REGEXP NULL;
+1 REGEXP NULL
+NULL
+End of 5.0 tests

mysql-test/t/func_regexp.test

@@ -74,4 +74,13 @@ execute stmt1 using @a;
 deallocate prepare stmt1;
 drop table t1;
 
-# End of 4.1 tests
+--echo End of 4.1 tests
+
+
+#
+# Bug #31440: 'select 1 regex null' asserts debug server
+#
+
+SELECT 1 REGEXP NULL;
+
+--echo End of 5.0 tests

sql/item_cmpfunc.cc

@@ -4285,6 +4285,7 @@ Item_func_regex::fix_fields(THD *thd, Item **ref)
     if (args[1]->null_value)
     {						// Will always return NULL
       maybe_null=1;
+      fixed= 1;
       return FALSE;
     }
     int error;