`IoReader::read()`: user-provided `Read` on uninitialized buffer may cause UB

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.

The flaw was fixed in commit 599313b by zero-initializing the buffer (via self.buf.resize(len, 0)) before passing it to Read.