SECURITY: pa_soa: Do not sign extend SOA 32-bit integer fields on 64-bit

SECURITY: pa_soa: Do not sign extend SOA 32-bit integer fields on 64-bit

The previous code used GET_W twice on two int variables, for no
explicable reason.  The top of these int variables was shifted left by
16, giving a 32-bit signed value.  The fields in adns_rr_soa are
unsigned long.  On a 64-bit machine, the 32-bit signed value is
implicitly sign extended.  This is entirely wrong.

Found by AFL 2.35b.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>