More error handling when joining channels with ChanKeep

modules/chankeep.py

@@ -136,6 +136,8 @@ def keepChannels(net, listinfo, mean, sigrelay, relay):
 
 def joinSingle(net, channel):
     eca = emptyChanAllocate(net, [channel], None, [])
+    if not eca:
+        return False
     if not len(eca.keys()) == 1:
         return False
     num = list(eca.keys())[0]