[Linux] Fix Xshm detection
[Linux] Fix Xshm detection

Error handler should be restored after XSync, or the new handler doesn't
take effect.

This fixes a crash when running graphics over ssh (if XSHM is built in).

giuliolunati/ren-c,draegtun/ren-c,rgchris/ren-c,hostilefork/rebol,kealist/ren-c,codebybrett/ren-c,codebybrett/ren-c,kealist/ren-c,mbk/ren-c,draegtun/ren-c,draegtun/ren-c,draegtun/ren-c,rgchris/ren-c,hostilefork/rebol,draegtun/ren-c,giuliolunati/ren-c,hostilefork/rebol,codebybrett/ren-c,kealist/ren-c,giuliolunati/ren-c,rgchris/ren-c,codebybrett/ren-c,kealist/ren-c,rgchris/ren-c,mbk/ren-c,mbk/ren-c,codebybrett/ren-c,hostilefork/rebol,mbk/ren-c,hostilefork/rebol,codebybrett/ren-c,giuliolunati/ren-c,rgchris/ren-c,kealist/ren-c,draegtun/ren-c,giuliolunati/ren-c,kealist/ren-c,hostilefork/rebol