Add a copy constructor to InvalidSass exception

The copy constructor transfers the ownership of `owned_src` from `rhs` to `lhs`.
Otherwise, `owned_src` may be destroyed too early and more than once.

In the libsass codebase, this copy can only happen on `throw`.

Modern compilers elide such copies. In our CI, only VS 2013 does not.

Author: glebm

src/error_handling.hpp

@@ -37,6 +37,11 @@ namespace Sass {
 
     class InvalidSass : public Base {
       public:
+        InvalidSass(InvalidSass& other) : Base(other) {
+          // Assumes that `this` will outlive `other`.
+          owned_src = other.owned_src;
+          other.owned_src = nullptr;
+        }
         InvalidSass(ParserState pstate, Backtraces traces, std::string msg, char* owned_src = nullptr);
         virtual ~InvalidSass() throw() { sass_free_memory(owned_src); };
         char *owned_src;