Fix minor issues within curl bindings

[BigBoyBarney]

slist_append:

https://curl.se/libcurl/c/curl_slist_append.html

Appends a string to a linked list of strings. The existing list should be passed as the first argument and the new list is returned from this function. Pass in NULL in the list argument to create a new list. The specified string has been appended when this function returns. curl_slist_append copies the string. The string argument must be a valid string pointer and cannot be NULL. [...]

multi_get_handles:

https://curl.se/libcurl/c/curl_multi_get_handles.html

Returns an array with pointers to all added easy handles. The end of the list is marked with a NULL pointer. [...]