avcodec/utils: Check that the data is complete in avpriv_bprint_to_extradata()

Fixes out of array read
Fixes: asan_heap-oob_4d2250_814_cov_2745172097_JACOsub_capability_tester.jss
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -3731,6 +3731,11 @@ int avpriv_bprint_to_extradata(AVCodecContext *avctx, struct AVBPrint *buf)
     ret = av_bprint_finalize(buf, &str);
     if (ret < 0)
         return ret;
+    if (!av_bprint_is_complete(buf)) {
+        av_free(str);
+        return AVERROR(ENOMEM);
+    }
+
     avctx->extradata = str;
     /* Note: the string is NUL terminated (so extradata can be read as a
      * string), but the ending character is not accounted in the size (in