winegcc: Don't call strendswith() with a NULL output_name.
Fixes: e3bf6e5e ("winegcc: Store all the boolean options in global variables.") Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=58006
Fixes: e3bf6e5e ("winegcc: Store all the boolean options in global variables.") Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=58006