spectrogram: convert int16 to unsigned for correct range

spectrogram: convert int16 to unsigned for correct range

p_dest is used to set the height of the column, and it is used
as a variable defined from zero to 2^16 - 1. It is therefore
considered in the rest of the program as an unsigned int 16.
Moreover, the value that we put inside are a sum of two squared
real value, so they are necessarily positive.

(cherry picked from commit 94a8d152eadd53073305c95d221eca7623b4ed6c)

Gbp-Pq: Name 0016-spectrogram-convert-int16-to-unsigned-for-correct-ra.patch