Buffer: fix out-of-bounds access in readInt/writeInt with zero byteLength

Buffer: fix out-of-bounds access in readInt/writeInt with zero byteLength

The variable-length readIntLE/UIntLE/BE and writeIntLE/UIntLE/BE only
rejected byteLength > 6.  byteLength == 0 passed, the bounds check
"size + index > byte_length" degenerated to "index > byte_length", and
switch (size) fell through to the 6-byte arm, reading or writing 6 bytes
past an attacker-chosen offset.  Require byteLength in [1, 6] as Node does.