Fix next_token() oob read. (closes #3787)

Fix next_token() oob read.  (closes #3787)

With specially crafted input to 'mutt -H', the line "Return-Path:<() "
is read and passed to mutt_parse_rfc822_line(). "<() " is then passed
through to rfc822_parse_adrlist().

Eventually, inside next_token(), is_special(*s) is called when s
points to the end of the string ('\0').  This macro calls strchr,
which will actually match and return a pointer to the trailing '\0' in
RFC822Specials!  This causes "s + 1" to be returned, skipping past the
end of string inside parse_mailboxdomain().

This patch adds a check to make sure *s is non-null before calling
is_special(*s).