lavfi: af_amerge options array was not ended with NULL

Add a NULL to prevent undefined behavior
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_amerge.c

@@ -49,6 +49,7 @@ typedef struct {
 static const AVOption amerge_options[] = {
     { "inputs", "specify the number of inputs", OFFSET(nb_inputs),
       AV_OPT_TYPE_INT, { .dbl = 2 }, 2, SWR_CH_MAX },
+    {0}
 };
 
 static const AVClass amerge_class = {