libX11: do not crash in GetResReq() macro
When _XGetRequest() detects that requested length exceeds remaining display output buffer capacity it would return NULL. GetResReq() macro obtains "req" pointer from a call to _XGetRequest() and then proceeds to assign request id through "req" pointer which leads to NULL pointer dereference in this case.
Fix this by checking if "req" is valid before assigning request id.
Closes #139 (closed)