eth/vm/logic/arithmetic: handle exp(X,0) == 1.

Caught by `exp8.json` test (not yet tracked in tests fixture).

Ref:

https://github.com/ethereum/tests/blob/10ab37c095bb87d2e781bcf112b6104912fccb44/VMTests/vmArithmeticTest/exp8.json

Accidentally caught in PR ethereum#1181.

Author: veox

eth/vm/logic/arithmetic.py

@@ -147,7 +147,9 @@ def exp(computation, gas_per_byte):
     bit_size = exponent.bit_length()
     byte_size = ceil8(bit_size) // 8
 
-    if base == 0:
+    if exponent == 0:
+        result = 1
+    elif base == 0:
         result = 0
     else:
         result = pow(base, exponent, constants.UINT_256_CEILING)