url: fix CURLOPT_POSTFIELDSIZE arg value check to allow -1

.. also add same arg value check to CURLOPT_POSTFIELDSIZE_LARGE.

Prior to this change since f121575c (7.56.1+) CURLOPT_POSTFIELDSIZE
erroneously rejected -1 value with CURLE_BAD_FUNCTION_ARGUMENT.

Bug: https://curl.haxx.se/mail/lib-2017-11/0000.html
Reported-by: Andrew Lambert