[Minor] spf: fix over-read on a bare "spf2." sender-id record

[Minor] spf: fix over-read on a bare "spf2." sender-id record

start_spf_parse validated only the "spf2." prefix (sizeof - 1) but
then advanced begin by the full sizeof, skipping one unvalidated
byte. A TXT record consisting of exactly "spf2." made the following
'/' check read past the logical end of the string, and could chain
into parse_spf_scopes walking past the allocation.

Advance past the validated prefix only, then check the version digit
and '/' with short-circuiting so neither read goes past the
terminator.