dir_scan: Avoid allocating new copies of buf_dir

This allocation is currently leaked, but as long as we don't allocate
new ones for each refresh, that shouldn't be much of an issue.

src/dir_scan.c

@@ -312,6 +312,7 @@ void dir_scan_init(const char *path) {
   dir_setlasterr(NULL);
   dir_seterr(NULL);
   dir_process = process;
+  if (!buf_dir)
     buf_dir = malloc(dir_memsize(""));
   pstate = ST_CALC;
 }