afs: Free afs_thiscell during shutdown

Currently, afs_thiscell can be allocated (via strdup) during client
startup, but is never freed. Free it in shutdown_cell() to avoid
leaking the memory.

Change-Id: I77954ef35f949c8a638ba15615148ab784f7f48f
Reviewed-on: https://gerrit.openafs.org/13714
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>

src/afs/afs_cell.c

@@ -1119,6 +1119,10 @@ shutdown_cell(void)
 	cn = next;
     }
 }
+    if (afs_thiscell) {
+	afs_osi_FreeStr(afs_thiscell);
+	afs_thiscell = NULL;
+    }
 }
 
 /*!