staging: dgap: fix possible NULL dereference

The return pointer from dgap_getword() is used in strcmp() where it is
dereferenced. But dgap_getword() can return NULL.
Lets put a check there and return 0 as error.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>