If a call to GetSystemDirectory fails, the `path` pointer that was
previously allocated would be leaked. This makes sure that `path` is
always freed.

Closes #938

LoadLibrary was supplanted by Curl_load_library for security
reasons in 6df916d7.

Inspiration provided by: Daniel Stenberg and Ray Satiro

Bug: https://curl.haxx.se/docs/adv_20160530.html

Ref: Windows DLL hijacking with curl, CVE-2016-4802