segmentation fault if batch has nully key or value

[dominictarr]

I was working a a PR to fix this in levelup, when I realized this was actually a leveldown problem.

[ralphtheninja]

I was just working with this :) Please assign yourself to an issue before you start working. That goes for myself too as well!

[ralphtheninja]

LOL, the fix was practically identical :) I also added a more generic macro and had the previous index-based version call the new one. I'm happy with this fix.

[rvagg]

Oh! Sorry! It's great to have other people getting dirty in the C++ btw.

[ralphtheninja]

The macros are a pain to debug though.

[rvagg]

I know... if you want to try an alternative approach then be my guest. I hate C macros but I also hate needless duplication.