Support pattern payload of size 0

[pstavirs]

The pattern payload protocol framesize is variable and is calculated based on the frameLength and the sum of all protocol frame sizes.

However, when building the frame value, for a calculated size of 0, pattern payload still adds one byte. This is due to this very old code in payload.cpp (not sure why it was added) -

                    // FIXME: Hack! Bad! Bad! Very Bad!!!
                    if (dataLen <= 0)
                        dataLen = 1;