http: fix build error on FreeBSD

http: fix build error on FreeBSD

The `result` parameter passed to `http_request_reauth()` may either
point to a `struct strbuf` or a `FILE *`, where the `target` parameter
tells us which of either it actually is. To accommodate for both types
the pointer is a `void *`, which we then pass directly to functions
without doing a cast.

This is fine on most platforms, but it breaks on FreeBSD because
`fileno()` is implemented as a macro that tries to directly access the
`FILE *` structure.

Fix this issue by storing the `FILE *` in a local variable before we
pass it on to other functions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>