avfilter/af_amerge: allow merging 1 input only

Useful when the amerge filter parameters are generated from a script based on
the number of input streams, by allowing 1 input it does not have to be handled
specially.

The split filter also allows 1 output, so it is more consistent to allow
merging 1 input as well.
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/af_amerge.c

@@ -52,7 +52,7 @@ typedef struct {
 
 static const AVOption amerge_options[] = {
     { "inputs", "specify the number of inputs", OFFSET(nb_inputs),
-      AV_OPT_TYPE_INT, { .i64 = 2 }, 2, SWR_CH_MAX, FLAGS },
+      AV_OPT_TYPE_INT, { .i64 = 2 }, 1, SWR_CH_MAX, FLAGS },
     { NULL }
 };