Fix panic on some messages
Fix panic on some messages

Some emails cause a panic. Minimal test case:

	Received: by 10.103.127.199 with HTTP; Tue, 29 Aug 2017 16:03:25 -0700 (PDT)
	Content-Type: multipart/alternative; boundary=3968118d3c1f8f73068bd950e51e704e052a58b415eb7d2e6c11c40caa51
	From: "example example" <example@gmail.com>

	--3968118d3c1f8f73068bd950e51e704e052a58b415eb7d2e6c11c40caa51
	Content-Transfer-Encoding: quoted-printable
	Content-Type: text/html

	<div dir=3D"auto">XXXXXXXXXXXXXX. Supprim=C3=A9e les.&nbsp;</div><div class=
	=3D"gmail_extra"><br><div class=3D"gmail_quote">Le&nbsp;29 ao=C3=BBt. 2017 =
	15:38, &quot;XXXXXX&quot; &lt;<a href=3D"mailto:xxxxxxx@example.com">s=
	xxxxxx@example.com</a>&gt; a =C3=A9crit&nbsp;<br>

	, XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.</i></font>

	--3968118d3c1f8f73068bd950e51e704e052a58b415eb7d2e6c11c40caa51--

This fix has been proposed twice before:

- https://github.com/saintienn/go-spamc/pull/5
- https://github.com/saintienn/go-spamc/pull/3

Teamwork/go-spamc,Teamwork/go-spamc,xepa/go-spamc,xepa/go-spamc