initialize ampl_out to avoid it containing garbage

static analysis from clang scan-build detected some paths
in the call to fsk_frame_analyze where ampl_out is not being
set:

fsk.c:493:13: warning: Assigned value is garbage or undefined
            best_a = ampl_out;

..probably the sanest approach here is to initialize it to 0.0
just to avoid it being garbage rather than setting it to zero in
the failed error paths in fsk_frame_analyze.

Signed-off-by: Colin Ian King <colin.king@canonical.com>