avfilter/avf_showspectrum: fix off by 1 error

Fixes: out of array access
Fixes: tickets/10749/poc15ffmpeg

Regression since: 81df787b53

Found-by: Zeng Yunxiang
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ab0fdaedd1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avf_showspectrum.c

@@ -1784,7 +1784,7 @@ static int showspectrumpic_request_frame(AVFilterLink *outlink)
             int acc_samples = 0;
             int dst_offset = 0;
 
-            while (nb_frame <= s->nb_frames) {
+            while (nb_frame < s->nb_frames) {
                 AVFrame *cur_frame = s->frames[nb_frame];
                 int cur_frame_samples = cur_frame->nb_samples;
                 int nb_samples = 0;