rxgen: Don't overflow PackageIndex

PackageIndex++ returns the pre-index value of PackageIndex, so the
error statement isn't run when PackageIndex == MAX_PACKAGES. This
means we go on to overflow all of the arrays that are MAX_PACKAGES
in size.

Caught by coverity (#985583, #985584, #985585, #985586,
                    #985587, #985588, #985589)

Reviewed-on: http://gerrit.openafs.org/9325
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit a1d8109c8fa8c10e3ce5ed67cda03b3b557608ff)

Change-Id: I5278bff5fe6be1bf127b240f7752c69385a9da24
Reviewed-on: http://gerrit.openafs.org/11033
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>