String alloc shouldn't convert to std::string.

As I work through making code use more C++ RAII and such, most
of the work is handling strings, especially temporaries.  As member
variables which manage string memory get turned into `std::string`,
some use cases might wind up leaking memory.  (One was found in
this change.)

By using a non-convertible-to-string result, such accidents should
be avoided.

src/form.cc

@@ -861,7 +861,7 @@ void Html_tag_open_option(DilloHtml *html, const char *tag, int tagsize)
       bool enabled = (a_Html_get_attr(html, tag, tagsize, "disabled") == NULL);
 
       auto option =
-         std::make_unique< DilloHtmlOption >(value ? dStrdup( value.value().c_str() ) : nullptr, label ? dStrdup( label.value().c_str() ) : nullptr, selected, enabled );
+         std::make_unique< DilloHtmlOption >(value ? dStrdup( value.value().c_str() ).ptr : nullptr, label ? dStrdup( label.value().c_str() ).ptr : nullptr, selected, enabled );
 
       input->select->addOpt( std::move( option ) );
    }