SUBSTR with negative argument didn't work

with multi-byte strings, length() instead
of numchars() where used in a mistake.

mysql-test/r/ctype_utf8.result

@@ -817,6 +817,9 @@ drop table t1;
 select 'c' like '\_' as want0;
 want0
 0
+SELECT SUBSTR('вася',-2);
+SUBSTR('вася',-2)
+ся
 create table t1 (id integer, a varchar(100) character set utf8 collate utf8_unicode_ci);
 insert into t1 values (1, 'Test');
 select * from t1 where soundex(a) = soundex('Test');

mysql-test/t/ctype_utf8.test

@@ -666,6 +666,12 @@ drop table t1;
 #
 select 'c' like '\_' as want0; 
 
+#
+# SUBSTR with negative offset didn't work with multi-byte strings
+#
+SELECT SUBSTR('вася',-2);
+
+
 #
 # Bug #7730 Server crash using soundex on an utf8 table
 #

sql/item_strfunc.cc

@@ -1023,7 +1023,7 @@ String *Item_func_substr::val_str(String *str)
   if ((null_value=(args[0]->null_value || args[1]->null_value ||
 		   (arg_count == 3 && args[2]->null_value))))
     return 0; /* purecov: inspected */
-  start= (int32)((start < 0) ? res->length() + start : start -1);
+  start= (int32)((start < 0) ? res->numchars() + start : start -1);
   start=res->charpos(start);
   length=res->charpos(length,start);
   if (start < 0 || (uint) start+1 > res->length() || length <= 0)