Submitted by 0xRajeev, also found by 0xsanson, cmichel, gpersoon, itsmeSTYJ, and pauliax.
The createMarket function allows accidental overwriting of previously created markets for the same combination of underlying and maturity timestamp (u, m) because there is no zero-address check to see if a previously created market exists for that combination.
Manual Analysis
Add a zero-address check for markets\[u]\[m] in createMarket before writing to it
