Allow reading/writing address 0 in the simulator (#1552)

Author: corollaries

pymodbus/datastore/simulator.py

@@ -554,8 +554,9 @@ def validate(self, func_code, address, count=1):
             # Bit count, correct to register count
             count = int((count + WORD_SIZE - 1) / WORD_SIZE)
             address = int(address / 16)
+
         real_address = self.fc_offset[func_code] + address
-        if real_address <= 0 or real_address > self.register_count:
+        if real_address < 0 or real_address > self.register_count:
             return False
 
         fx_write = func_code in self._write_func_code