wp_tempnam produces filenames longer than 255 characters

[doems]

Summary
Since the maximal filename length of the most filesystems out there is 255, the function should try to avoid filenames longer than 255 characters.

Reproduce
If the passed-in $filename parameter is longer than 245 characters (WordPress appends a string which has 10 characters), PHP will fail on the following line: ​https://github.com/WordPress/WordPress/blob/6995ec613e5f113cedea4f0481b9030f79fabc0a/wp-admin/includes/file.php#L183.

Fix

<?php
if ( strlen( $filename ) > 245 ) {
    $filename = substr( $filename, 0, 244);
}

[obenland]

Introduced in [35644] for #34562.