gzip: don't use an uninitialized file descriptor

gzip: don't use an uninitialized file descriptor

gzip interprets an argument of "-" as indicating stdin, but when
"-" is not the first name on the command line, it doesn't work.
* gzip.c (treat_stdin): Initialize "ifd".
Patch and fine description by Lasse Collin in
http://www.mail-archive.com/bug-gzip@gnu.org/msg00213.html
* Makefile.am (check-local): Exercise the fix.  Based on the above.
* NEWS (Bug fixes): Mention this.