common: Fix an uninitialized variable

This could be uninitialized if the app has --socket=fallback-x11 but
not --socket=wayland, although that's a combination that doesn't make
a great deal of sense. Regression introduced during recent refactoring.

Fixes: db7a8bb5 "common: Split up socket setup from flatpak-run into multiple files"
Signed-off-by: Simon McVittie <smcv@collabora.com>
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