Some server in the wild do not put the boundary at a newline
as rfc1347 7.2.1 states.
Cope with that by reading a line and if it is not empty reading
a second one.
Reported-By: bitingsock
The first check is done without the AVIOContext, so alloc it only if said check succeeds
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Janne Grunau <janne-libav@jannau.net>