EIP-210: Fixes for blockhash contracts (#1094)

* EIP-210: Fix blockhash contract for block number 1

This fixes the issue in the contract when executed from SYSTEM account at block 1. Then the number of the previous block is 0 and the contract executes the loop forever.

* EIP-210: Correctly handle argument with negative value

Fix fixes the issue in the blockhash contract where the negative value of the argument is not properly handled.

EIPS/eip-210.md

@@ -49,13 +49,13 @@ The Serpent source code is:
 with offset = 0:
     if msg.sender == 0xfffffffffffffffffffffffffffffffffffffffe:
         with bn = block.number - 1:
-            while 1:
+            while bn:
                 ~sstore(offset + ~mod(bn, 256), ~calldataload(0))
                 if ~mod(bn, 256):
                     ~stop()
                 bn = ~div(bn, 256)
                 offset += 256
-    elif ~calldataload(0) < block.number:
+    elif ~calldataload(0) >= 0 and ~calldataload(0) < block.number:
         with tbn = ~calldataload(0):
             with dist_minus_one = block.number - tbn - 1:
                 while dist_minus_one >= 256 && ~mod(tbn, 256) == 0:
@@ -72,13 +72,13 @@ with offset = 0:
 The EVM init code is:
 
 ```
-0x6100e28061000e6000396100f056600073fffffffffffffffffffffffffffffffffffffffe33141561005957600143035b60011561005357600035610100820683015561010081061561004057005b6101008104905061010082019150610022565b506100e0565b4360003512156100d4576000356001814303035b61010081121515610085576000610100830614610088565b60005b156100a75761010083019250610100820491506101008104905061006d565b610100811215156100bd57600060a052602060a0f35b610100820683015460c052602060c0f350506100df565b600060e052602060e0f35b5b505b6000f3
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
 ```
 
 The EVM bytecode that the contract code should be set to is:
 
 ```
-0x600073fffffffffffffffffffffffffffffffffffffffe33141561005957600143035b60011561005357600035610100820683015561010081061561004057005b6101008104905061010082019150610022565b506100e0565b4360003512156100d4576000356001814303035b61010081121515610085576000610100830614610088565b60005b156100a75761010083019250610100820491506101008104905061006d565b610100811215156100bd57600060a052602060a0f35b610100820683015460c052602060c0f350506100df565b600060e052602060e0f35b5b50
+0x600073fffffffffffffffffffffffffffffffffffffffe33141561005857600143035b801561005257600035610100820683015561010081061561003f57005b6101008104905061010082019150610022565b506100f3565b600060003512151561006e574360003512610071565b60005b156100e7576000356001814303035b6101008112151561009857600061010083061461009b565b60005b156100ba57610100830192506101008204915061010081049050610080565b610100811215156100d057600060a052602060a0f35b610100820683015460c052602060c0f350506100f2565b600060e052602060e0f35b5b50
 ```
 
 ### Rationale