gzip: fix bug with any upper case custom ('-S'-specified) suffix

gzip: fix bug with any upper case custom ('-S'-specified) suffix

Any user-specified suffix with an upper case
letter would fail to match desired file.
* gzip.c (get_suffix): First, arrange to have only one return
point rather than two. Put a lower-cased (just-malloc'd) copy
of z_suffix in the suffix vector, and free it before returning.
* tests/upper-suffix: New file.
* tests/Makefile.am (TESTS): Add it.
* NEWS: Mention it.
Reported by meo@xenialab.it in https://bugs.gnu.org/29006