Improve robustness of Con_CenterPrintf()

Improve robustness of Con_CenterPrintf()

This code was OK, but if someone were to change the buffer size(s) such
that line[] were smaller than msg[], or were to call Con_CenterPrintf()
with a maxLineLength > 40, then a buffer overflow would have become
possible.  This patch prevents it.

See https://github.com/DarkPlacesEngine/darkplaces/pull/159

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>