virt-login-shell: silence coverity warning

Coverity spotted that 'nfdlist' (ssize_t) could be -1, but that we
were using 'i' (size_t) to iterate over the list at cleanup, with
crashing results because it promotes to a really big unsigned number.

* tools/virt-login-shell.c (main): Avoid treating -1 as unsigned.
Signed-off-by: NEric Blake <eblake@redhat.com>

tools/virt-login-shell.c

@@ -339,8 +339,9 @@ main(int argc, char **argv)
     /* At this point, the parent is now waiting for the child to exit,
      * but as that may take a long time, we release resources now.  */
 cleanup:
-    for (i = 0; i < nfdlist; i++)
-        VIR_FORCE_CLOSE(fdlist[i]);
+    if (nfdlist > 0)
+        for (i = 0; i < nfdlist; i++)
+            VIR_FORCE_CLOSE(fdlist[i]);
     VIR_FREE(fdlist);
     virConfFree(conf);
     if (dom)