Fix std::bitset overflow

Fixed #166

Author: vitalybuka

src/mutator.cc

@@ -55,7 +55,7 @@ enum class Mutation : uint8_t {
   Last = Clone,
 };
 
-using MutationBitset = std::bitset<static_cast<size_t>(Mutation::Last)>;
+using MutationBitset = std::bitset<static_cast<size_t>(Mutation::Last) + 1>;
 
 using Messages = std::vector<Message*>;
 using ConstMessages = std::vector<const Message*>;