avfilter/geq: unbreak alpha processing

d607af50 broke alpha processing by always overwriting any supplied
alpha expression.

libavfilter/vf_geq.c

@@ -275,7 +275,7 @@ static av_cold int geq_init(AVFilterContext *ctx)
 
     if (!geq->expr_str[A] && geq->bps != 32) {
         geq->expr_str[A] = av_asprintf("%d", (1<<geq->bps) - 1);
-    } else {
+    } else if (!geq->expr_str[A]) {
         geq->expr_str[A] = av_asprintf("%f", 1.f);
     }
     if (!geq->expr_str[G])