Fix broken build by using ADVANCE_CHAR macro return.

We forget to check or use the return value of the ADVANCE_CHAR macro,
and upon compilation an error is thrown because of its lack of use. This
patch checks to see if the macro was successful, and if not replaces the
offending character with a replacement.

json_tokener.c

@@ -507,9 +507,13 @@ struct json_object* json_tokener_parse_ex(struct json_tokener *tok,
                       (str[1] == '\\') &&
                       (str[2] == 'u'))
                   {
-	            ADVANCE_CHAR(str, tok);
-	            ADVANCE_CHAR(str, tok);
-
+                /* Advance through the 16 bit surrogate, and move on to the
+                 * next sequence. The next step is to process the following
+                 * characters.
+                 */
+	            if( !ADVANCE_CHAR(str, tok) || !ADVANCE_CHAR(str, tok) ) {
+                    printbuf_memappend_fast(tok->pb, (char*)utf8_replacement_char, 3);
+                }
                     /* Advance to the first char of the next sequence and
                      * continue processing with the next sequence.
                      */