[5.1] Fix improper use of mb_substr()

[vlakoff]

Follow-up to #10894.
Because strpos($path, '?') is single-byte, the subsequent substr() have to be single-byte as well.

• url('ééé/ààà?ùùù', 'extra');
  • http://example.org/ééé/ààà/extra?ùùù – expected
  • http://example.org/ééé/ààà?ùùù/extra – result
• url('ééé?ààà?ùùù', 'extra');
  • http://example.org/ééé/extra?ààà?ùùù – expected
  • http://example.org/ééé?àà/extraà?ùùù – result

ping @rkgrep :)

[vlakoff]

In case you would be interested, the following implementation is about 65% faster (10000 iterations: from 147 ms to 51 ms), and imho clearer:

if (strpos($path, '?') !== false) {
    $parts = explode('?', $path, 2);
    $path = $parts[0];
    $query = '?'.$parts[1];
} else {
    $query = '';
}