We're recently trying to upgrade to the current mater of libgit2 in Cargo but
we're unfortunately hitting a segfault in one of our tests. A particular test is
just a small smoke test that https works (e.g. it's configured in libgit2). This
test attempts to clone from a URL which simply immediately drops connections
after they're accepted (e.g. terminate abnormally). We expect to see a standard
error from libgit2 but unfortunately we're seeing a segfault.

This segfault is happening inside of the `wait_for` function of `curl_stream.c`
at the line `FD_SET(fd, &errfd)` because `fd` is -1. This ends up doing an
out-of-bounds array access that faults the program. I tracked back to where this
-1 came from to the line here (returned by `CURLINFO_LASTSOCKET`) and added a
check to return an error. This may not be the right fix though, so feedback is
appreciated!