Fix use-after-free in lsof_select_process_regex

Set xp to NULL after assigning to CmdRx[i].exp so that CLEAN(xp) in
the cleanup path does not free memory that is now owned by CmdRx[i].exp.

Author: cuiweixie

lib/lsof.c

@@ -926,6 +926,7 @@ enum lsof_error lsof_select_process_regex(struct lsof_context *ctx, char *x) {
      */
     CmdRx[i].mc = 0;
     CmdRx[i].exp = xp;
+    xp = NULL;
     NCmdRxU++;
 
     /** Update selection flags for inclusion */