libafs: don't free a null pointer in an unlikely error condition

libafs: don't free a null pointer in an unlikely error condition

It is extremely unlikely that we will ever fail to allocate two
bytes in SRXAFSCB_GetCellServDB() to hold the empty-string return
value for the case where the specified cell can't be found.  But
that would result in freeing a null pointer, so check for it.

Change-Id: I47a296148e231b0ef20ecd18b8458b912f22a58c
Found-by: clang static analyzer with the help of AFS_NONNULL
Reviewed-on: http://gerrit.openafs.org/5261
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>