Secured the WPM and RPM service encoders (#604)

* Secured the WPM and RPM client service encoders.

* Fixed RPM and WPM apps when fail to encode request. 

* Fixed WPM app number of arguments checking.

src/bacnet/basic/service/s_wpm.c

@@ -92,8 +92,10 @@ uint8_t Send_Write_Property_Multiple_Request(uint8_t *pdu,
         /* encode the APDU portion of the packet */
         len = wpm_encode_apdu(
             &pdu[pdu_len], max_pdu - pdu_len, invoke_id, write_access_data);
+        if (len <= 0) {
+            return 0;
+        }
         pdu_len += len;
-
         /* will it fit in the sender?
            note: if there is a bottleneck router in between
            us and the destination, we won't know unless