test the protocol id for 0

previous test would fail if one byte was non zero.

src/modbus-tcp.c

@@ -188,6 +188,7 @@ static int _modbus_tcp_check_integrity(modbus_t *ctx, uint8_t *msg, const int ms
 static int _modbus_tcp_pre_check_confirmation(modbus_t *ctx, const uint8_t *req,
                                               const uint8_t *rsp, int rsp_length)
 {
+    unsigned int protocol_id;
     /* Check transaction ID */
     if (req[0] != rsp[0] || req[1] != rsp[1]) {
         if (ctx->debug) {
@@ -199,10 +200,11 @@ static int _modbus_tcp_pre_check_confirmation(modbus_t *ctx, const uint8_t *req,
     }
 
     /* Check protocol ID */
-    if (rsp[2] != 0x0 && rsp[3] != 0x0) {
+    protocol_id = (rsp[2] << 8) + rsp[3];
+    if (protocol_id != 0x0) {
         if (ctx->debug) {
             fprintf(stderr, "Invalid protocol ID received 0x%X (not 0x0)\n",
-                    (rsp[2] << 8) + rsp[3]);
+                    protocol_id);
         }
         errno = EMBBADDATA;
         return -1;