Fixed a bug in the Add UGen which caused  each output buffer to have a
constant value (i.e. b[i] = b[0] for 1 <= i < bufferSize) when Add was
used to add a constant value to a signal. It appears that this bug was
one cause (perhaps the main one) of the clicking/noise artefacts that
were introduced when the delay time (TapOut UGen) was modulated, since
usually one would add a constant to the modulation signal to ensure the
delay time never goes negative.