Default constructor for CScriptWitness so clang doesn't complain.

Author: CodeShark

src/script/script.h

@@ -624,6 +624,9 @@ struct CScriptWitness
 {
     std::vector<std::vector<unsigned char> > stack;
 
+    // Some compilers complain without a default constructor
+    CScriptWitness() { }
+
     bool IsNull() const { return stack.empty(); }
 };