strip: Don't generate empty output file when nothing to do.

strip: Don't generate empty output file when nothing to do.

If there was nothing to do strip would skip generating a separate
debug file if one was requested, but it would also not finish the
creation of a new output file (with the non-stripped sections).
Also if there was an error any partially created output would be kept.

Make sure that when the -o output file option is given we always generate
a complete output file (except on error). Also make sure that when the -f
debug file option is given it is only generated when it is not empty.

Add testcase run-strip-nothing.sh that tests the various combinations.

https://sourceware.org/bugzilla/show_bug.cgi?id=21522

Signed-off-by: Mark Wielaard <mark@klomp.org>