sanitize max_fee_rate; fixes #2663

lib/simple_config.py

@@ -199,7 +199,10 @@ class SimpleConfig(PrintError):
             self.set_key('gui_last_wallet', path)
 
     def max_fee_rate(self):
-        return self.get('max_fee_rate', MAX_FEE_RATE)
+        f = self.get('max_fee_rate', MAX_FEE_RATE)
+        if f==0:
+            f = MAX_FEE_RATE
+        return f
 
     def dynfee(self, i):
         if i < 4: