new.block< old.block for zero price feed.

[harsh-98]

when for a token price feed is changed to zero price feed, Price for that token is added while syncing priceOracle. as a result, zero priceFeed is added first, and price events from previous pricefeed are yet to be synced. So, check (old.block< new.block) fails.
Solution:
Ignore this check if the saved price event has 0 price. In such case, last price block number can be greater than new price event being added.

a0db78c fix.