Expanding a binary field should result in 0x00-filled positions, not 0x20

(ASCII space).  For Bug#16857.

mysql-test/r/binary.result

@@ -141,3 +141,22 @@ t1	CREATE TABLE `t1` (
   `a` binary(1) default NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+create table t1 (col1 binary(4));
+insert into t1 values ('a'),('a ');
+select hex(col1) from t1;
+hex(col1)
+61000000
+61200000
+alter table t1 modify col1 binary(10);
+select hex(col1) from t1;
+hex(col1)
+61000000000000000000
+61200000000000000000
+insert into t1 values ('b'),('b ');
+select hex(col1) from t1;
+hex(col1)
+61000000000000000000
+61200000000000000000
+62000000000000000000
+62200000000000000000
+drop table t1;

mysql-test/t/binary.test

@@ -89,3 +89,15 @@ show create table t1;
 drop table t1;
 
 # End of 4.1 tests
+
+#
+# Bug#16857
+#
+create table t1 (col1 binary(4));
+insert into t1 values ('a'),('a ');
+select hex(col1) from t1;
+alter table t1 modify col1 binary(10);
+select hex(col1) from t1;
+insert into t1 values ('b'),('b ');
+select hex(col1) from t1;
+drop table t1;

sql/field_conv.cc

@@ -379,6 +379,16 @@ static void do_cut_string_complex(Copy_field *copy)
 
 
 
+static void do_expand_binary(Copy_field *copy)
+{
+  CHARSET_INFO *cs= copy->from_field->charset();
+  memcpy(copy->to_ptr,copy->from_ptr,copy->from_length);
+  cs->cset->fill(cs, copy->to_ptr+copy->from_length,
+                     copy->to_length-copy->from_length, '\0');
+}
+
+
+
 static void do_expand_string(Copy_field *copy)
 {
   CHARSET_INFO *cs= copy->from_field->charset();
@@ -583,8 +593,14 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
 	return (from->charset()->mbmaxlen == 1 ?
                 do_cut_string : do_cut_string_complex);
       else if (to_length > from_length)
+      {
+        if ((to->flags & BINARY_FLAG) != 0)
+          return do_expand_binary;
+        else
           return do_expand_string;
       }
+
+    }
     else if (to->real_type() != from->real_type() ||
 	     to_length != from_length ||
              !compatible_db_low_byte_first)