osc.c: Fix do_fft() to never set output data to -Inf value
 - This prevents gtkdatabox from hanging when such values are being displayed.
 - Configurations such as: FMComms2 in alert mode or FMComms2 with digital loopback set while scale is set to -Inf are are known to cause such problems.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>