avfilter/af_alimiter: Check nextpos before use

Fixes: out of array read
Fixes: tickets/10744/poc11ffmpeg

Found-by: Li Zeyuan and Zeng Yunxiang.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a88b06f9ee)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_alimiter.c

@@ -195,6 +195,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
                     int j = i % buffer_size;
                     double ppeak = 0, pdelta;
 
+                    if (nextpos[j] >= 0)
                         for (c = 0; c < channels; c++) {
                             ppeak = FFMAX(ppeak, fabs(buffer[nextpos[j] + c]));
                         }