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.

dpi/cookies.cc

@@ -1329,7 +1329,7 @@ static std::string Cookies_get(char *url_host, char *url_path,
    int i;
 
    if (disabled)
-      return dStrdup("");
+      return "";
 
    matching_cookies = dList_new(8);
 

dpi/datauri.cc

@@ -274,7 +274,7 @@ static unsigned char *datauri_get_data(char *url, size_t *p_sz)
          data = (unsigned char *)a_Url_decode_hex_str(p, p_sz);
       }
    } else {
-      data = (unsigned char *)dStrdup("");
+      data = (unsigned char *)dStrdup("").ptr;
       *p_sz = 0;
    }