[token-2022] zero amount processing - HAL-07

[samkim-crypto]

A number of token-2022 program functions allow for zero amount processing:

• process_transfer
• process_approve
• process_mint_to
• process_burn
• process_deposit
• process_withdraw

Unnecessary computations can be avoided if these functions check the parameter before the main logic.

[joncinque]

Be careful, a lot of these are actually by design -- we used to no-op for zero amounts, but that creates false positives for incorrect instructions. For example, if we return earlier, I can successfully "transfer" 0 tokens from your account to mine without a signature. So be sure that all of the ownership checks and all that are still performed.