Use checked arithmetic for TZif data block lengths

[n0liu]

Addresses the TZif parser finding in #1802.

State::new in the TZif parser computes several data-block lengths by multiplying counts taken straight from the (untrusted) header — transition_count * time_size, type_count * 6, leap_count * (time_size + 4). A malformed header with a huge count overflows the usize multiplication, which panics on 32-bit targets. (On 64-bit the product still fits, and the oversized read_exact fails cleanly, so the panic is 32-bit only.)

Compute those lengths with checked_mul and return Error::InvalidTzFile on overflow, so a malformed header is rejected instead of panicking.

Added a test that feeds a header with type_count = u32::MAX and asserts parsing returns an error. Full test suite, cargo fmt, and clippy -D warnings pass.