reuse_conn(): free old_conn->options

This fixes a memory leak when CURLOPT_LOGIN_OPTIONS is used, together with
connection reuse.

I found this with oss-fuzz on GDAL and curl master:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=9582
I couldn't reproduce with the oss-fuzz original test case, but looking
at curl source code pointed to this well reproducable leak.

Closes #2790