libstand: tftp_open() can leak pkt on error

The memory can be leaked if we will have pkt set and will get an error
during tftp_open() processing.

Differential Revision:	https://reviews.freebsd.org/D12202

lib/libstand/tftp.c

@@ -467,6 +467,7 @@ tftp_open(const char *path, struct open_file *f)
 
 	if (res) {
 		free(tftpfile->path);
+		free(tftpfile->pkt);
 		free(tftpfile);
 		return (res);
 	}