comedi: me_daq: Fix potential overrun of firmware buffer

In the Linux kernel, the following vulnerability has been resolved:

comedi: me_daq: Fix potential overrun of firmware buffer

`me2600_xilinx_download()` loads the firmware that was requested by

`request_firmware()`. It is possible for it to overrun the source

buffer because it blindly trusts the file format. It reads a data

stream length from the first 4 bytes into variable `file_length` and

reads the data stream contents of length `file_length` from offset 16

onwards. Although it checks that the supplied firmware is at least 16

bytes long, it does not check that it is long enough to contain the data

stream.

Add a test to ensure that the supplied firmware is long enough to

contain the header and the data stream. On failure, log an error and

return `-EINVAL`.