Alpao asdkGetLastError message is not a null-terminated string

[carandraug]

From Python Microscope code comments for alpao deformable mirrors:

    ## asdkGetLastError should write a null-terminated string but
    ## doesn't seem like it (at least CannotOpenCfg does not ends in
    ## null) so we empty the buffer ourselves before using it.  Note
    ## that even when there are no errors, we need to empty the buffer
    ## because the buffer has the message 'No error in stack'.