[5.4] Avoid unneeded function call in Str::length()

[vlakoff]

Follow-up to #19047.

$nb = 10000;

$t1 = microtime(true);
for ($i = $nb; $i--; ) {
    mb_strlen('éééààà', mb_internal_encoding());
}
$t2 = microtime(true);
for ($i = $nb; $i--; ) {
    mb_strlen('éééààà');
}
$t3 = microtime(true);

echo $t2 - $t1, "\n", $t3 - $t2;

before 116ms, after 46ms