[BUG] Boundschecks

[lkdvos]

Currently, no bounds checking is ever happening:

A = SparseArrayDOK{Int}(2,2)
A[3,3] # 0

This is rather surprising and can lead to very annoying to track bugs.
Similarly, no checks are done on constructed sparse arrays' storage either.

[mtfishman]

Good point, I was meaning to get back to that once things with the design settled down more, now would be a good time to add that in.