http: Check for AVERROR_EOF in the check for premature end

http: Check for AVERROR_EOF in the check for premature end

When the check was added (in 3668701f9600, in 2015), some IO
functions returned 0 on EOF (in particular, the TCP protocol
did, but the TLS protocol returned AVERROR_EOF). Since
0e1f771d2200d in 2017, the TCP protocol also returns AVERROR_EOF
instead of 0, making the check for premature end never have the
intended effect.

Signed-off-by: Martin Storsjö <martin@martin.st>