fix lfn ff handle leak

deltree must restart ff on backtrack in SFN mode - see 7e3365b.
In LFN mode there are ff handles that require findnext() until
completion, or they are leaked. So close them explicitly.

Regression of 7e3365b - LFNs were not enabled back then.

Note: it is possible to special-case LFN mode and not restart
ff, relying on open handles. But this means coding difference
with SFN mode, and also there may be insufficient ff handles
in a redirector to traverse the large dir tree.

src/command.c

@@ -2419,6 +2419,17 @@ static void perform_deltree(const char *arg)
             }
           if (subdir_level > 0 || remove_level1_dir)
             {
+            /* Close cur dir when entering next level. We need this
+             * for 2 reasons:
+             * 1. In SFN mode there are no handles, so ff process needs
+             *    to be restarted on backtrack (see 7e3365bd).
+             *    To unify with LFN mode, we restart in both cases.
+             * 2. Even special-casing LFN mode may not be safe as it
+             *    has the limited amount of ff handles.
+             * So just close handle and restart later. For SFN, close
+             * is a no-op.
+             */
+            findclose_f(ffhandle);
             subdir_level++;
             if (subdir_level >= MAX_SUBDIR_LEVEL)
               {