afs_fetchstore: avoid use of uninitialized variable

rxfs_fetchInit() attempts to do a 64-bit RPC first, but falls back
to the 32-bit StartRXAFS_FetchData() if the server appears to not
support the 64-bit RPCs.

We correctly did not read a length from the call if the FetchData
RPC(s) failed, but proceeded to assign from the 'length' local
variable into the 'alength' output variable unconditionally later on.

Instead of blindly continuing on, jump to the error-handling part of
the routine when we cannot read a length from the call.  This has the
side effect of skipping an afs_Trace3() point in the error case.

Reviewed-on: http://gerrit.openafs.org/10694
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit baf6af8a8f2207ce39b746d59ca4bc661c002883)

Change-Id: Icf14d5e8a6abf8a8a014ab7d48b767e3dcc7a6a9
Reviewed-on: http://gerrit.openafs.org/10742
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>