fix(electrum): Handle negative heights properly

fix(electrum): Handle negative heights properly

In electrum, heights returned alongside txs may be 0 or -1. 0 means the
tx is unconfirmed. -1 means the tx is unconfirmed and spends an
unconfirmed tx.

Previously, the codebase assumed that heights cannot be negative and
used a `i32 as usize` cast (which would lead to panic if the i32 is
negative).