Fixes problem with set multiple Register/Coils

[Laserlicht]

No description provided.

[blddk]

There still seems to be some kind of problem with multiple items.
The first two return their status correct, both for coils and inputs. Input registers seems to work fine.

Coils and inputs returns their status at high.

[volkerdh]

Please read my comment to this issue in the comment section. By moving the section for building the response after the section for writing to registers you might by no löonger compliant to Modbus error handling as you might respond with an error (due to impossible malloc() for the response) although having already successfully changed Modbus register content. I like to suggest going a different way (described in my comment to this issue in the comment section): We should better use a second independent frame buffer for input and thus enable to malloc() the output frame buffer for the response before changing Modbus registers.