afs_fetchstore: re-avoid uninitialized variable

As noted in the gerrit comments for change 10742, commit
baf6af8a8f2207ce39b746d59ca4bc661c002883 does not handle the case
where the second rx_Read() call fails, and the 'length' variable
can still be used uninitialized.

Instead of using an err label and jumping to it on the case of
errors, initialize length to zero and take care to neither
set nor access *alength if an error has occurred.  This is
more consistent with the style of the surrounding code while still
avoiding the use of an uninitialized variable.

Change-Id: I6abfa4a5f051368ca12ada1494fc7687f378d319