dumptool: Remove newlines safely

The code currently does

   fgets(cmdbuf, ... );
   cmdbuf[strlen(cmdbuf - 1)] = '\0';

in order to remove new lines from cmdbuf. Coverity thinks there's
a danger of strlen(cmdbuf) being 0, and thus the strlen being negative.
That shouldn't happen, but if fgets hits EOF midway through a line, we
might get a string that doesn't have a trailing '\n', and end up
removing the wrong character. Tidy this up by checking that the string
isn't 0 length, and that the character we're zapping is a newline.

Caught by coverity (#985430)

Reviewed-on: http://gerrit.openafs.org/9310
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 8f51502e5f45a43fba130d260813716be894d51e)

Change-Id: Ie165c8e50c4071c07fffa220601c8b5e92ccf815
Reviewed-on: http://gerrit.openafs.org/11026
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>