[XembedSNIProxy] Do not crash on null pointer

Summary:
XCB may return null pointer as a response. Add a check to prevent
segmentation fault.

BUG: 409652
FIXED-IN: 5.17.1

Test Plan: It is hard to reproduce, but the reason of the segmentation fault is pretty obvious, thanks to the debug dump from bug 409652

Reviewers: #plasma_workspaces, #plasma, davidedmundson

Reviewed By: #plasma_workspaces, #plasma, davidedmundson

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24514