avfilter/af_afir: clamp initial ir selection

the ir option may exceed nbirs because its static range is independent
of the configured number of impulse responses. init() only clamps
prev_selir, leaving selir able to index ctx->inputs out of range.

with the default nbirs=1, afir=ir=1 reads ctx->inputs[2] and crashes.
clamp selir like process_command() and add a fate test.

Signed-off-by: Ayoub Nabil Boubagrat <237098474+ayoubnabil@users.noreply.github.com>

libavfilter/af_afir.c

@@ -663,7 +663,9 @@ static av_cold int init(AVFilterContext *ctx)
     AVFilterPad pad;
     int ret;
 
-    s->prev_selir = FFMIN(s->nb_irs - 1, s->selir);
+    /* ir is bounded by nbirs, not by its static AVOption range. */
+    s->selir      = FFMIN(s->nb_irs - 1, s->selir);
+    s->prev_selir = s->selir;
 
     pad = (AVFilterPad) {
         .name = "main",