Protocol handling issues in libXv - CVE-2016-5407

The Xv query functions for adaptors and encodings suffer from out of
boundary accesses if a hostile X server sends a maliciously crafted
response.

A previous fix already checks the received length against fixed values
but ignores additional length specifications which are stored inside
the received data.

These lengths are accessed in a for-loop. The easiest way to guarantee
a correct processing is by validating all lengths against the
remaining size left before accessing referenced memory.

This makes the previously applied check obsolete, therefore I removed
it.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>