BUG/MINOR: proxy: free orgto_hdr_name in free_proxy()

Unlike fwdfor_hdr_name, orgto_hdr_name was not properly freed in
free_proxy().

This did not cause observable memory leaks because originalto proxy option is
only used for user configurable proxies, which are solely freed right before
process termination.
No backport needed unless some architectural changes causing regular proxies
to be freed and reused multiple times within single process lifetime are made.

src/proxy.c

@@ -350,6 +350,7 @@ void free_proxy(struct proxy *p)
 
 	free(p->desc);
 	istfree(&p->fwdfor_hdr_name);
+	istfree(&p->orgto_hdr_name);
 
 	task_destroy(p->task);