Xcb: Check if geom is valid before using it

It looks like xcb_get_geometry_reply() may
return a null geom without returning an error,
so geom has to be null pointer checked.

Bug: chromium:1237736
Change-Id: I8bafecae3cf0a57e7073405e817b9b3175f61a41
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/56288
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Alexis Hétu <sugoi@google.com>

src/WSI/XcbSurfaceKHR.cpp

diff --git a/src/WSI/XcbSurfaceKHR.cpp b/src/WSI/XcbSurfaceKHR.cpp
index fa03b36..2e9fa36 100644
--- a/src/WSI/XcbSurfaceKHR.cpp
+++ b/src/WSI/XcbSurfaceKHR.cpp
@@ -96,7 +96,7 @@
 	{
 		free(error);
 	}
-	else
+	else if(geom)
 	{
 		windowExtent.width = static_cast<uint32_t>(geom->width);
 		windowExtent.height = static_cast<uint32_t>(geom->height);