Fix for FixedHash::operator~().

libdevcore/FixedHash.h

@@ -100,7 +100,7 @@ public:
 	FixedHash operator|(FixedHash const& _c) const { return FixedHash(*this) |= _c; }
 	FixedHash& operator&=(FixedHash const& _c) { for (unsigned i = 0; i < N; ++i) m_data[i] &= _c.m_data[i]; return *this; }
 	FixedHash operator&(FixedHash const& _c) const { return FixedHash(*this) &= _c; }
-	FixedHash& operator~() { for (unsigned i = 0; i < N; ++i) m_data[i] = ~m_data[i]; return *this; }
+	FixedHash operator~() const { FixedHash ret; for (unsigned i = 0; i < N; ++i) ret[i] = ~m_data[i]; return ret; }
 
 	/// @returns true if all bytes in @a _c are set in this object.
 	bool contains(FixedHash const& _c) const { return (*this & _c) == _c; }