[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