Fix Oscillation Flag in Stable2.sol

[nickkatsios]

• Stable2 calculation of LP Token Supply identifies stable oscillations of 2 units during the Newtonian Estimation loop to prevent unnecessary iterations using the stableOscillation flag. The flag, when enabled should break the loop, returning the average estimation.
• This PR moves the flag out of the loop declaration so that it does not get reinitialized upon every iteration.