[SX1276, LoRaWAN] Fix confusing return value in LoRaWAN::processJoinAccept (#1262)

The error RADIOLIB_ERR_LORA_HEADER_DAMAGED board returned fromreadData(...)
and signaled from a SX1276 is in fact correctly ignored but the state
variable is not corrected.

This confuses the caller due to the wrongly returned state value
RADIOLIB_ERR_LORA_HEADER_DAMAGED.

Change: Clear state also.

src/protocols/LoRaWAN/LoRaWAN.cpp

@@ -688,6 +688,8 @@ int16_t LoRaWANNode::processJoinAccept(LoRaWANJoinEvent_t *joinEvent) {
   // we can ignore that error
   if(state != RADIOLIB_ERR_LORA_HEADER_DAMAGED) {
     RADIOLIB_ASSERT(state);
+  } else {
+    state = RADIOLIB_ERR_NONE;
   }
 
   // check reply message type