Fix VD-1301 and VD-1302 vulnerabilities

This patch was contributed by Maor Vermucht and Or Peles from
VDOO Connected Trust.

src/modbus.c

@@ -839,9 +839,10 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
         break;
     case MODBUS_FC_WRITE_MULTIPLE_COILS: {
         int nb = (req[offset + 3] << 8) + req[offset + 4];
+        int nb_bits = req[offset + 5];
         int mapping_address = address - mb_mapping->start_bits;
 
-        if (nb < 1 || MODBUS_MAX_WRITE_BITS < nb) {
+        if (nb < 1 || MODBUS_MAX_WRITE_BITS < nb || nb_bits * 8 < nb) {
             /* May be the indication has been truncated on reading because of
              * invalid address (eg. nb is 0 but the request contains values to
              * write) so it's necessary to flush. */
@@ -870,9 +871,10 @@ int modbus_reply(modbus_t *ctx, const uint8_t *req,
         break;
     case MODBUS_FC_WRITE_MULTIPLE_REGISTERS: {
         int nb = (req[offset + 3] << 8) + req[offset + 4];
+        int nb_bytes = req[offset + 5];
         int mapping_address = address - mb_mapping->start_registers;
 
-        if (nb < 1 || MODBUS_MAX_WRITE_REGISTERS < nb) {
+        if (nb < 1 || MODBUS_MAX_WRITE_REGISTERS < nb || nb_bytes * 8 < nb) {
             rsp_length = response_exception(
                 ctx, &sft, MODBUS_EXCEPTION_ILLEGAL_DATA_VALUE, rsp, TRUE,
                 "Illegal number of values %d in write_registers (max %d)\n",