Fixing a bug in case of a Optional Route Subpattern

As described in #16 (comment)

src/Router/Core.php

@@ -378,7 +378,7 @@ private static function findRoute(
                 // Extract the matched URL parameters (and only the parameters)
                 $params = array_map(function ($match, $index) use ($matches) {
                     // We have a following parameter: take the substring from the current param position until the next one's position (thank you PREG_OFFSET_CAPTURE)
-                    if (isset($matches[$index + 1]) && isset($matches[$index + 1][0]) && is_array($matches[$index + 1][0])) {
+                    if (isset($matches[$index + 1]) && isset($matches[$index + 1][0]) && $matches[$index + 1][0][1] != -1 && is_array($matches[$index + 1][0])) {
                         return trim(substr($match[0][0], 0, $matches[$index + 1][0][1] - $match[0][1]), '/');
                     }