urlapi: return NULL from curl_url_dup() when given a NULL handle

[filtede98]

Passing a NULL handle to curl_url_dup() caused a NULL pointer dereference crash inside the DUP() macro when evaluating (in)->scheme.

Return NULL immediately if !in is passed, matching curl_easy_duphandle() and preventing crashes when duplicating uninitialized or failed handles.

Also verify this in test1560 and update curl_url_dup(3).

[testclutch]

Analysis of PR #23051 at fc806f09:

Test ../../tests/http/test_40_socks.py::TestSocks::test_40_03_dl_serial[h2-socks5] failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

Generated by Testclutch

[bagder]

Thanks!