The withdraw_excess_lamports function does not prevent self-transfers

In the process_transfer function, which is responsible for transferring tokens from one account to another, there is a self-transfer check. This check compares the source and destination account to see if they are the same account, which would be a self-transfer that will be treated differently to a regular transfer.

Another function that has the ability to move tokens is the withdraw_excess_lamports function. This function is missing a check to see if the source and destination account are matching, thus allowing self-transfers. If these self-transfers are undesirable, a similar check to the process_transfer one can be added to the withdraw_excess_lamports function.