staging: r8188eu: get a string from the user correctly

The original code had two bugs:
1) It didn't check if the string was zero length so it could oops when
   it tried to dereference the ZERO_SIZE_PTR.
2) It didn't enforce that the string was NUL terminated.

It was also messy as pants.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>