Bug#13832953 MY_STRNXFRM_UNICODE: ASSERTION `SRC' FAILED. | MY_STRNXFRM_UNICODE/MAKE_SORTKEY

Problem:

Field_set::val_str in case of an empty SET value
returned a String with str_length==0 and Ptr==0,
which is not expected by some pieces of the code.

Fix:
      
Returning an empty string with str_length==0 and Ptr=="",
like Field_enum does.

mysql-test/r/ctype_ucs.result

@@ -4107,5 +4107,16 @@ DROP TABLE t1;
 SET sql_mode=default;
 SET NAMES latin1;
 #
+# Bug #13832953 	MY_STRNXFRM_UNICODE: ASSERTION `SRC' FAILED
+#
+CREATE TABLE t1 (c1 SET('','') CHARACTER SET ucs2);
+Warnings:
+Note	1291	Column 'c1' has duplicated value '' in SET
+INSERT INTO t1 VALUES ('');
+SELECT COALESCE(c1) FROM t1 ORDER BY 1;
+COALESCE(c1)
+
+DROP TABLE t1;
+#
 # End of 5.5 tests
 #

mysql-test/t/ctype_ucs.test

@@ -759,6 +759,15 @@ SET collation_connection=ucs2_general_ci;
 --source include/ctype_numconv.inc
 SET NAMES latin1;
 
+--echo #
+--echo # Bug #13832953 	MY_STRNXFRM_UNICODE: ASSERTION `SRC' FAILED
+--echo #
+CREATE TABLE t1 (c1 SET('','') CHARACTER SET ucs2);
+INSERT INTO t1 VALUES ('');
+SELECT COALESCE(c1) FROM t1 ORDER BY 1;
+DROP TABLE t1;
+
+
 --echo #
 --echo # End of 5.5 tests
 --echo #

sql/field.cc

@@ -8257,8 +8257,7 @@ String *Field_set::val_str(String *val_buffer,
   ulonglong tmp=(ulonglong) Field_enum::val_int();
   uint bitnr=0;
 
-  val_buffer->length(0);
-  val_buffer->set_charset(field_charset);
+  val_buffer->set("", 0, field_charset);
   while (tmp && bitnr < (uint) typelib->count)
   {
     if (tmp & 1)