Fix moodbars not generating correctly

Fixes #4635

The magnitude output from gstfastspectrum was not initialized for the first FFT.
In some cases, the memory would contain data that upon math operations becomes NaN.
This causes one or more colours of the moodbar to be generated blank, resulting in missing colours or just plain black moodbars.
Can you see if adding parenthesis after the new double[bands] works as well?
Like: spectrum->spect_magnitude = new double[bands]();

This should value-initialise the items in the array to 0.

If that doesn't work maybe try with {} instead - that should be C++11 uniform initialization.