Fix transport recv return value

* Transport interface recv function returns 0 should represent the
  read operation can be retried. Negative value should be returned
  when input parameters and recv bytes are invalid,

libraries/coreMQTT/port/network_transport/network_transport.c

@@ -202,7 +202,7 @@ int32_t espTlsTransportSend( NetworkContext_t* pxNetworkContext,
 int32_t espTlsTransportRecv( NetworkContext_t* pxNetworkContext,
                              void* pvData, size_t uxDataLen )
 {
-    int32_t lBytesRead = 0;
+    int32_t lBytesRead = -1;
 
     if( ( pvData != NULL ) &&
         ( uxDataLen > 0 ) &&
@@ -222,6 +222,8 @@ int32_t espTlsTransportRecv( NetworkContext_t* pxNetworkContext,
             fd_set read_fds;
             fd_set error_fds;
 
+            lBytesRead = 0;
+
             esp_tls_get_conn_sockfd( pxNetworkContext->pxTls, &lSockFd );
             FD_ZERO( &read_fds );
             FD_SET( lSockFd, &read_fds );