Description:
------------
If a MEDIUMBLOB column has an empty value (length of zero), 
mysqli_stmt_bind_result() attempts to allocate a buffer of 16M 
for it. The offending code is in mysqli_api.c, line 332:

if (stmt->stmt->fields[ofs].max_length == 0) {

This will be true if the column is empty as well as in the 
case where the user has not called store_result(). The result 
is the code using the value of length instead of max_length, 
which is 16M for a mediumblob.

Reproduce code:
---------------
Assume there exists a table:
CREATE TABLE a_table ( some_blob MEDIUMBLOB NOT NULL );

<?php

$m = new mysqli( MY_DB_HOST, MY_DB_USER, MY_DB_PASS );
$s = new mysqli_stmt( $m, "SELECT some_blob FROM a_table WHERE some_blob='' LIMIT 1" );
$s->execute();
$s->store_result();
print_r( $s->result_metadata()->fetch_fields() );
$s->bind_result( $data );
$s->fetch();
print_r( $data );

?>


Expected result:
----------------
Array
(
    [0] => stdClass Object
        (
            [name] => some_blob
            [orgname] => some_blob
            [table] => a_table
            [orgtable] => a_table
            [def] => 
            [max_length] => 0
            [length] => 0
            [charsetnr] => 63
            [flags] => 144
            [type] => 252
            [decimals] => 0
        )

)


Actual result:
--------------
Array
(
    [0] => stdClass Object
        (
            [name] => some_blob
            [orgname] => some_blob
            [table] => a_table
            [orgtable] => a_table
            [def] => 
            [max_length] => 0
            [length] => 16777215
            [charsetnr] => 63
            [flags] => 144
            [type] => 252
            [decimals] => 0
        )

)

Fatal error: Allowed memory size of 8388608 bytes exhausted 
(tried to allocate 16777216 bytes) in test.php on line 8