Don't dereference null pointers for apn, userName, password

[metadaddy]

Omitting apn, userName, passWord arguments from call to GPRS::join resulted in errors due to dereferencing the default null values (long string of garbage data was sent to device). Simply test apn, userName, passWord against null before sending them.

[ortegafernando]

Hi,

Have you checked it ? I mean, do you have any sim card that works without APN or USERNAME or PASSWORD ?

Is it ok for you ?

Hope your reply.

[metadaddy]

Yes - I have a SIM that works with only apn, no username or password. It would fail when passing only apn (username, password default to 0, so it would pass garbage to the device). Adding the tests fixes it. I added apn test as well since there is a default parameter for apn also, and dereferencing a null pointer is not a good thing to do!

[ortegafernando]

Thanks a lot, merged.

[metadaddy]

Great - thanks!