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>