avcodec/g2meet: Fix framebuf size

Currently the code can in some cases draw tiles that hang outside the
allocated buffer. This patch increases the buffer size to avoid out
of array accesses. An alternative would be to fail if such tiles are
encountered.
I do not know if any valid files use such hanging tiles.

Fixes Ticket2971
Found-by: ami_stuff
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>