Fix for bug#6317: string function CHAR, parameter is NULL, wrong result

mysql-test/r/func_str.result

@@ -733,3 +733,9 @@ WHERE a = CONV('e251273eb74a8ee3', 16, 10);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	
 DROP TABLE t1;
+SELECT CHAR(NULL,121,83,81,'76') as my_column;
+my_column
+ySQL
+SELECT CHAR_LENGTH(CHAR(NULL,121,83,81,'76')) as my_column;
+my_column
+4

mysql-test/t/func_str.test

@@ -476,3 +476,9 @@ EXPLAIN
   WHERE a = CONV('e251273eb74a8ee3', 16, 10);
 
 DROP TABLE t1;
+
+#
+# Bug #6317: string function CHAR, parameter is NULL, wrong result
+#
+SELECT CHAR(NULL,121,83,81,'76') as my_column;
+SELECT CHAR_LENGTH(CHAR(NULL,121,83,81,'76')) as my_column;

sql/item_strfunc.cc

@@ -1868,6 +1868,7 @@ String *Item_func_char::val_str(String *str)
   {
     int32 num=(int32) args[i]->val_int();
     if (!args[i]->null_value)
+    {
 #ifdef USE_MB
       if (use_mb(collation.collation))
       {
@@ -1884,6 +1885,7 @@ b1:        str->append((char)(num>>8));
 #endif
       str->append((char)num);
     }
+  }
   str->set_charset(collation.collation);
   str->realloc(str->length());			// Add end 0 (for Purify)
   return str;