fix(p/grc20): Unsafe casting during grc20 minting

[notJoon]

Description

The totalSupply could overflow when casting from uint64 to int64. If totalSupply exceeded 2^63-1 (max value for int64), the cast would produce a negative value, which when cast back to uint64 would result in an invalid totalSupply.

To fix this, modified Mint function to check for overflow before adding to totalSupply. Added explicit overflow checks to prevent any potential overflow issues.

[Gno2D2]

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