avformat/http: check the auth string contents not the pointer which cannot be NULL

It appears this bug originates from a "work in progress" patch from
ffmpeg-devel that was heavily redesigned by and integrated in libav

And that patch even had a reply and review on the mailing list pointing
out that it had a bug.

This fixes a deadlock with ffserver

See: [FFmpeg-devel] [PATCH] Fix HTTP authentication problem for POST actions.
     [FFmpeg-devel] [PATCH 1/3] Introduce auth_phase flag, which will be true if authorization needs to be sent, but the type of authorization is not known yet Partial fix #3036
     [FFmpeg-devel] [PATCH 2/3] Only add Transfer-Encoding header when not in authorization phase, because server will wait (indefinitely) for data when receiving this header Partial fix #3036
     [FFmpeg-devel] [PATCH 3/3] Only allow posting data and/or forcing a 200 code, enabling posting isml chunks, -after- we did a possible first request to get a 403 from the server telling us which type of authentication to apply Final part fix #3036
See: 71549a857b13edf4c4f95037de6ed5bb4c4bd4af
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
