When stsp changed bwi_encap() for 30 bit addresses in rev 1.106 code

When stsp changed bwi_encap() for 30 bit addresses in rev 1.106 code
that used to always initialise the error variable is no longer run.

And at the end of bwi_encap() there is:

	if (error)
		m_freem(m);
	return (error);

Fixing this prevents packet loss stsp was seeing.

ok stsp@ miod@ deraadt@