ext/intl: Fix UConverter::transcode() substitution length truncation

[LamentXU123]

Now, in UConverter::transcode() we masked the from_subst and to_subst option string length with 0x7F before passing them to ucnv_setSubstChars(). This makes every string with length greater than 127 be silently truncated.

For example, a string with length 129 will now be truncated as with length 1.

POC:

<?php
$subst = str_repeat('A', 129);

var_dump(UConverter::transcode('abc', 'UTF-8', 'ASCII', ['from_subst' => $subst]));
var_dump(intl_get_error_code());
var_dump(UConverter::transcode('abc', 'UTF-8', 'ASCII', ['to_subst' => $subst]));
var_dump(intl_get_error_code());
?>

Will return

string(3) "abc"
int(0)
string(3) "abc"
int(0)

While I suggest to directly reject strings longer than 0x7F, that is

bool(false)
int(1)
bool(false)
int(1)

Reproduce

Intentionally targeting master as this is technically not a bug fix.

[devnexen]

is indeed a correct diagnostic otherwise.