Fixed Array.prototype.lastIndexOf() with unicode string as "this".

Previously, when lastIndexOf() was called with unicode string as "this"
argument and a negative "fromIndex" argument null-pointer dererence
might occur because njs_string_offset() was called with invalid index
value whereas njs_string_offset() should always be called with valid
index argument.

The fix is to verify that from index is valid.

This closes #482 issue on Github.