fix usb communications when receiving empty frame

E.g. before this fix, iso15 (and probably others) raw commands
expecting results but with no answer from the tag where showing the
previously received commands instead of : "[!] /!\ command failed"

client/src/comms.c

@@ -414,6 +414,15 @@ __attribute__((force_align_arg_pointer))
                         }
                     }
                 }
+                else if ((!error) && (length == 0)) { // we received an empty frame
+                    if (rx.ng)
+                        rx.length = 0; // set received length to 0
+                    else {  // old frames can't be empty
+                        PrintAndLogEx(WARNING, "Received empty MIX packet frame (length: 0x00)");
+
+                        error = true;
+                    }
+                }
                 if (!error) {                        // Get the postamble
                     res = uart_receive(sp, (uint8_t *)&rx_raw.foopost, sizeof(PacketResponseNGPostamble), &rxlen);
                     if ((res != PM3_SUCCESS) || (rxlen != sizeof(PacketResponseNGPostamble))) {