Avoid negative block numbers, ref #649.

EIPS/eip-649.md

@@ -22,7 +22,7 @@ The Casper development and switch to proof-of-stake is delayed, the Ethash proof
 #### Relax Difficulty with Fake Block Number
 For the purposes of `calc_difficulty`, simply replace the use of `block.number`, as used in the exponential ice age component, with the formula:
 
-    fake_block_number = min(0, block.number - 3_000_000) if block.number >= BYZANTIUM_FORK_BLKNUM else block.number
+    fake_block_number = max(0, block.number - 3_000_000) if block.number >= BYZANTIUM_FORK_BLKNUM else block.number
 
 #### Adjust Block, Uncle, and Nephew rewards
 To add an issuance reduction, adjust the block reward to `new_block_reward`, where