When receiving data from the other end in usbmux mode, we need to return any
data we get, not wait to fill the input buffer.

Otherwise any clients waiting for data will wait forever: in fact the initial
handshake would fail to complete, because we wouldn't pass along the 28-byte
success response because we were trying to fill the 1028-byte buffer with
data.

Also add some input validation to make sure we don't write data into random
memory + take the requested offset into account when writing data into the
output array.