Fix 0 amount bug in spend_allowance

[brson]

This fixes a low severity bug in a token contract in this repo.

This repo contains code copied from the Soroban example token,
code which recieved a fix in the upstream repo that has not been applied here.

The fix in question, from December, is in this commit:

stellar/soroban-examples@c794712#diff-02d9e91ec617d641664acc20ed164964099f2b96a099109d42cdbb80d44087aeR56

From reading, I think the main consequence of this bug is that a
previously-unauthorized spender calling transfer_from can write a 0 to the
temporary storage for their own allowance; though it also seems to allow
unintended behavior like allowing authorized spenders to extend the ttl of the
allowance without transfering.

As far as I know this bug does not have a security impact.

[spiraldevp]

Looked it over, makes sense, merging the PR