Fix empty payload checksum being appended even if payload is empty.

TinyFrame.c

@@ -537,6 +537,7 @@ void _TF_FN TF_AcceptChar(TinyFrame *tf, unsigned char c)
                 }
 
                 if (tf->len == 0) {
+                    // if the message has no body, we're done.
                     TF_HandleReceivedMessage(tf);
                     TF_ResetParser(tf);
                     break;
@@ -773,6 +774,8 @@ static bool _TF_FN TF_SendFrame(TinyFrame *tf, TF_Msg *msg, TF_Listener listener
         }
     }
 
+    // Checksum only if message had a body
+    if (msg->len > 0) {
         // Flush if checksum wouldn't fit in the buffer
         if (TF_SENDBUF_LEN - len < sizeof(TF_CKSUM)) {
             TF_WriteImpl(tf, (const uint8_t *) tf->sendbuf, len);
@@ -780,9 +783,10 @@ static bool _TF_FN TF_SendFrame(TinyFrame *tf, TF_Msg *msg, TF_Listener listener
         }
 
         // Add checksum, flush what remains to be sent
-    len += TF_ComposeTail(tf->sendbuf+len, &cksum);
-    TF_WriteImpl(tf, (const uint8_t *) tf->sendbuf, len);
+        len += TF_ComposeTail(tf->sendbuf + len, &cksum);
+    }
 
+    TF_WriteImpl(tf, (const uint8_t *) tf->sendbuf, len);
     TF_ReleaseTx(tf);
 
     return true;