Make code more robust to OOM attacks

[Enet4]

I found a few more edge cases through fuzzing (just in case, the fuzz project is currently in the fuzz branch). Not all cases of out-of-memory (OOM) attacks can be solved, but this will at least solve the trivial (yet very problematic) case of OOM by attempting to read a file which claims to have an extremely large size via dim.

Summary:

• Check for arithmetic overflow when calculating the size of the volume.
• Only pre-allocate volumes and extension data up to a sane size to prevent trivial OOM attacks.
• Add 3 fuzz test cases for InMemNiftiObject that used to lead to panics or even aborts (!).

[Enet4]

This looks good to go. @nilgoyette As usual, thank you for the review.