avfilter/xpsnr: avoid division by zero

The ref input may have its frame rate unset, which would then lead to
SIGFPE. So fall back to the main link frame rate. If that too is unset,
default to 0.

Related to #11428

Author: GyanD

libavfilter/vf_xpsnr.c

@@ -552,6 +552,7 @@ static int config_input_ref(AVFilterLink *inlink)
     AVFilterContext  *ctx = inlink->dst;
     XPSNRContext *const s = ctx->priv;
     FilterLink *il = ff_filter_link(inlink);
+    FilterLink *ml = ff_filter_link(ctx->inputs[0]);
 
     if ((ctx->inputs[0]->w != ctx->inputs[1]->w) ||
         (ctx->inputs[0]->h != ctx->inputs[1]->h)) {
@@ -568,7 +569,9 @@ static int config_input_ref(AVFilterLink *inlink)
     s->max_error_64 = (1 << s->depth) - 1; /* conventional limit */
     s->max_error_64 *= s->max_error_64;
 
-    s->frame_rate = il->frame_rate.num / il->frame_rate.den;
+    // Avoid division by zero
+    s->frame_rate = il->frame_rate.den ? (il->frame_rate.num / il->frame_rate.den) :
+                    ml->frame_rate.den ? (ml->frame_rate.num / ml->frame_rate.den) : 0;
 
     s->num_comps = (desc->nb_components > 3 ? 3 : desc->nb_components);