Funds stuck if not claimed before liquidation

Summary

Funds can get stuck permanently if they arent claimed before liquidation.

Vulnerability Detail

If WETH get withdrawn from alchemix and gets marked as claimable, every share gets assigned a a portion of it to be claimed:

@internal
def _mark_as_claimable(_amount: uint256):
    """
    @notice
        Marks _amount of WETH as claimable by token holders and
        calculates the amount_claimable_per_share.
    """
    if _amount == 0 or self.total_shares == 0:
        return

    assert ERC20(WETH).balanceOf(self) >= _amount

    self.amount_claimable_per_share += _amount * PRECISION / self.total_shares

If someone liquidates his shares before claiming the WETH that he was eligible to, they will be locked in the contract forever. They could be potentially retrieved via a migration, which currently is not working though and which is not the purpose of the migration mechanism either.

A simple example:

There are 100 shares and after some time 100 WETH are withdrawn from alchemix. Now every share can claim 1 WETH.
A user liquidates his 20 shares without claiming his fraction of WETH from step 1 first. Now there are only 80 shares, each eligible to still claim 1 WETH each. 20 WETH from the 100 above will be stuck in the contract.