avfilter/avf_showfreqs: Use floating point division in WFUNC_BHANN

Fixes: CID1322365
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avf_showfreqs.c

@@ -208,7 +208,7 @@ static void generate_window_func(float *lut, int N, int win_func, float *overlap
         break;
     case WFUNC_BHANN:
         for (n = 0; n < N; n++)
-            lut[n] = 0.62-0.48*FFABS(n/(N-1)-.5)-0.38*cos(2*M_PI*n/(N-1));
+            lut[n] = 0.62-0.48*FFABS(n/(double)(N-1)-.5)-0.38*cos(2*M_PI*n/(N-1));
         *overlap = 0.5;
         break;
     case WFUNC_SINE: