zgrep -<NUM> doesn't work if <NUM> is two digits or greater

[Thomas Bushnell, BSG]

Create a gzipped file:

   $ for i in $(seq 1 100); do echo $i; done > file.txt
   $ gzip file.txt

Then:

   $ zgrep -9 17 file.txt.gz
   8
   9
   ...
   25
   26

works.  But:

   $ zgrep -10 17 file.txt.gz
   gzip: 17.gz: No such file or directory

fails.

Note that "grep -9 17 file.txt" and "grep -10 17 file.txt" work just
fine.

(https://bugs.launchpad.net/bugs/1032831)

Please include me in the CC on any replies.

Thomas