avcodec/libaribcaption: allocate one rect in text mode

aribcaption_trans_text_subtitle() always fills exactly one rect, but
allocated the rects array with ctx->caption.region_count entries. For a
caption without regions, such as a clear screen, that is a zero-sized
allocation, and writing sub->rects[0] goes past its end.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DdDiDRusmi6eD7qSrKBjHb
Found-by: Joshua Rogers <joshua.rogers@aisle.com>

libavcodec/libaribcaption.c

@@ -814,7 +814,7 @@ static int aribcaption_trans_text_subtitle(ARIBCaptionContext *ctx)
     int ret = 0;
     const char *text;
 
-    sub->rects = av_calloc(ctx->caption.region_count, sizeof(*sub->rects));
+    sub->rects = av_calloc(1, sizeof(*sub->rects));
     if (!sub->rects) {
         ret = AVERROR(ENOMEM);
         goto fail;