reftable/record: don't abort when decoding invalid ref value type

reftable/record: don't abort when decoding invalid ref value type

When decoding a ref record we read its value type from the block. In
case the type itself is invalid we call `abort()`. This is rather
heavy-handed though: the data we're reading is untrusted, so we should
treat the issue as a normal and not as a programming error.

Fix this by handling the error gracefully. Note that this also requires
us to set the value type later, as otherwise we might store an invalid
type in the record.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>