comedi: me4000: Fix potential overrun of firmware buffer

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

comedi: me4000: Fix potential overrun of firmware buffer

`me4000_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.

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`.

Note: The firmware loading was totally broken before commit ac584af59945

("staging: comedi: me4000: fix firmware downloading"), but that is the

most sensible target for this fix.