[SNMP4J] Notification filtering

[Matthias Wiesmann]

Hello again.

I got notification filtering to work (with my test case, at least).

I had to correct several problems.
- the getRows works as per my previous post.
- idxSnmpNotifyFilterSubtree now is 1
- entries in the vbMatch array are allocated as needed, but the last
matching loop did not check if an entry was not null before trying to
get its size, hence the null reference exception. This is corrected
now.

In order to understand the code, I added some comments and cleaned up
the function a little bit, for instance the subtreeWithLength variable
is now created using the MOTableIndex method, instead of
re-implementing the same functionality inline.

In only tested with an empty mask string. I corrected the mask
function in my previous post so that it handle empty mask strings, but
I doubt that mask expansion is handled correctly in the OID class.

Cheers

Matthias Wiesmann