Bug#14255 CAST(x AS BINARY(N)) does not pad

type_binary.result, type_binary.test:
  Adding test case.
item_timefunc.cc:
  Padding code was added.

mysql-test/r/type_binary.result

@@ -111,3 +111,6 @@ select count(distinct s1) from t1;
 count(distinct s1)
 3
 drop table t1;
+select hex(cast(0x10 as binary(2)));
+hex(cast(0x10 as binary(2)))
+1000

mysql-test/t/type_binary.test

@@ -65,3 +65,6 @@ select hex(s1) from t1 where s1=0x0120;
 select hex(s1) from t1 where s1=0x0100;
 select count(distinct s1) from t1;
 drop table t1;
+
+# check that cast appends trailing zeros
+select hex(cast(0x10 as binary(2)));

sql/item_timefunc.cc

@@ -2327,8 +2327,9 @@ String *Item_char_typecast::val_str(String *str)
     and the result is longer than cast length, e.g.
     CAST('string' AS CHAR(1))
   */
-  if (cast_length >= 0 &&
-      (res->length() > (length= (uint32) res->charpos(cast_length))))
+  if (cast_length >= 0)
+  {
+    if (res->length() > (length= (uint32) res->charpos(cast_length)))
     {                                           // Safe even if const arg
       char char_type[40];
       my_snprintf(char_type, sizeof(char_type), "CHAR(%lu)", length);
@@ -2344,6 +2345,19 @@ String *Item_char_typecast::val_str(String *str)
                           res->c_ptr_safe());
       res->length((uint) length);
     }
+    else if (cast_cs == &my_charset_bin && res->length() < (uint) cast_length)
+    {
+      if (res->alloced_length() < (uint) cast_length)
+      {
+        str->alloc(cast_length);
+        str->copy(*res);
+        res= str;
+      }
+      bzero((char*) res->ptr() + res->length(),
+            (uint) cast_length - res->length());
+      res->length(cast_length);
+    }
+  }
   null_value= 0;
   return res;
 }