Fix regression when reading Event::PaymentReceived in some cases

For some reason rustc was deciding on a type for the `Option` being
deserialized for us as `_user_payment_id`. This really, really,
absolutely should have been a compile failure - the type (with
methods called on it!) was ambiguous! Instead, rustc seems to have
been defaulting to `Option<()>`, causing us to read zero of the
eight bytes in the `user_payment_id` field, which returns an
`Err(InvalidValue)` error as TLVs must always be read fully.

This should likely be reported to rustc as its definitely a bug,
but I cannot seem to cause the same error on any kinda of
vaguely-minimized version of the same code.

Found by `chanmon_consistency` fuzz target.

Author: TheBlueMatt

lightning/src/util/events.rs

@@ -476,7 +476,7 @@ impl MaybeReadable for Event {
 					let mut payment_preimage = None;
 					let mut payment_secret = None;
 					let mut amt = 0;
-					let mut _user_payment_id = None; // For compatibility with 0.0.103 and earlier
+					let mut _user_payment_id = None::<u64>; // For compatibility with 0.0.103 and earlier
 					read_tlv_fields!(reader, {
 						(0, payment_hash, required),
 						(2, payment_secret, option),