fix error when no header requested

Author: jerryneedell

adafruit_rfm69.py

@@ -782,7 +782,7 @@ def receive(self, timeout=0.5, keep_listening=True, with_header=False,
             if (rx_filter != _RH_BROADCAST_ADDRESS and packet[0] != _RH_BROADCAST_ADDRESS
                     and packet[0] != rx_filter):
                 packet = None
-            if not with_header:  # skip the header if not wanted
+            elif not with_header:  # skip the header if not wanted
                 packet = packet[4:]
 
         # Listen again if necessary and return the result packet.