Maximum gas logic doesn't account for overflows

[ValarDragon]

This is another currently exploitable that will happen w.r.t. to gas wanted. The app can send multiple gas wanteds on the order of 2**62 or 2**63. Addition of these can cause overflow into a very small positive value. Then tendermint will have proposed a block with a much larger amount of gas than the gas limit. This is a problem both in the mempool, block validation logic, and existing cosmos application logic.

• Fix overflow in Mempool Reap
• Fix overflow in block validation before precommitting

I'll make a separate issue for the SDK.

[ValarDragon]

This problem didn't actually exist due to the way we structured the > -1 checks! The existing problem was fixed by #2994