modbus_reply: fix copy & paste error in sanity check (fixes #614)

While handling MODBUS_FC_WRITE_AND_READ_REGISTERS, both address offsets
must be checked, i.e. the read and the write address must be within the
mapping range.

At the moment, only the read address was considered, it looks like a
simple copy and paste error, so let's fix it.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

src/modbus.c

@@ -961,7 +961,7 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
                 nb_write, nb, MODBUS_MAX_WR_WRITE_REGISTERS, MODBUS_MAX_WR_READ_REGISTERS);
         } else if (mapping_address < 0 ||
                    (mapping_address + nb) > mb_mapping->nb_registers ||
-                   mapping_address < 0 ||
+                   mapping_address_write < 0 ||
                    (mapping_address_write + nb_write) > mb_mapping->nb_registers) {
             rsp_length = response_exception(
                 ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_ADDRESS, rsp, FALSE,