Fix compile on Visual Studio

libevm/VM.h

@@ -427,12 +427,12 @@ template <class Ext> dev::bytesConstRef dev::eth::VM::go(Ext& _ext, OnOpFunc con
 			break;
 		case Instruction::SIGNEXTEND:
 		{
-				unsigned k = m_stack.back();
+			unsigned k = m_stack.back().convert_to<unsigned>();
 			m_stack.pop_back();
 			auto& b = m_stack.back();
 			if (k <= 31)
 				if ((b >> (k * 8)) & 0x80)
-					for (int i = 31; i > k; --i)
+					for (unsigned i = 31; i > k; --i)
 						b |= (u256(0xff) << i);
 			break;
 		}