uncompress: if the source is broken, don't delete the target

Fixes the second half of PR bin/19722, reported by Giorgos Keramidas.

uncompress: if the source doesn't exist, don't touch the target

Fixes part of PR bin/19722, reported by Giorgos Keramidas.

tests/compress: extend tests for compressing and uncompressing

For PR bin/19722.

tests/compress: demonstrate truncation of target file

Reported by Giorgos Keramidas in PR#19722.