X11 use uint16_t for the width/height of things. Anything too large will
be truncated when sending the request to the X11 server. This commit
adds a size check to a function that did not check things and then later
caused a segmentation fault.

Not adding a test case because the test case from the below bug report
allocates 3,5 GiB of memory, which I find too much for a test.

Fixes: #414


Signed-off-by: Uli Schlachter <psychon@znc.in>