GHSA-jwh2-vrr9-vcp2: mz-avro's incorrect use of `set_len` allows for un-initialized memory

Affected versions of this crate passes an uninitialized buffer to a user-provided Read implementation.

Arbitrary Read implementations can read from the uninitialized buffer (memory exposure) and also can return incorrect number of bytes written to the buffer. Reading from uninitialized memory produces undefined values that can quickly invoke undefined behavior.

Note: there is only UB in the case where a user provides a struct whose Read implementation inspects the buffer passed to read_exact before writing to it. This is an unidiomatic (albeit possible) Read implementation.

See https://github.com/MaterializeInc/materialize/issues/8669 for details.