avfilter/af_amerge: avoid undefined shift (<<64) in outlayout setup

Fixes CID1322306
Reviewed-by: Ganesh Ajjanagadde <gajjanag@mit.edu>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_amerge.c

@@ -110,8 +110,8 @@ static int query_formats(AVFilterContext *ctx)
         for (i = 0; i < nb_ch; i++)
             s->route[i] = i;
         outlayout = av_get_default_channel_layout(nb_ch);
-        if (!outlayout)
-            outlayout = ((int64_t)1 << nb_ch) - 1;
+        if (!outlayout && nb_ch)
+            outlayout = 0xFFFFFFFFFFFFFFFFULL >> (64 - nb_ch);
     } else {
         int *route[SWR_CH_MAX];
         int c, out_ch_number = 0;