lavc/utils: make sure the iconv descriptor can be opened only once.

This prevents trying to do some subtitles conversion for each event when
the character encoding is not found. It now aborts early instead of
flooding stderr.

libavcodec/utils.c

@@ -1106,12 +1106,23 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
                 if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_AUTOMATIC)
                     avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_PRE_DECODER;
 
-                if (!CONFIG_ICONV && avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
+                if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
+#if CONFIG_ICONV
+                    iconv_t cd = iconv_open("UTF-8", avctx->sub_charenc);
+                    if (cd == (iconv_t)-1) {
+                        av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context "
+                               "with input character encoding \"%s\"\n", avctx->sub_charenc);
+                        ret = AVERROR(errno);
+                        goto free_and_end;
+                    }
+                    iconv_close(cd);
+#else
                     av_log(avctx, AV_LOG_ERROR, "Character encoding subtitles "
                            "conversion needs a libavcodec built with iconv support "
                            "for this codec\n");
                     ret = AVERROR(ENOSYS);
                     goto free_and_end;
+#endif
                 }
             }
         }
@@ -1899,12 +1910,7 @@ static int recode_subtitle(AVCodecContext *avctx,
 
 #if CONFIG_ICONV
     cd = iconv_open("UTF-8", avctx->sub_charenc);
-    if (cd == (iconv_t)-1) {
-        av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context "
-               "with input character encoding \"%s\"\n", avctx->sub_charenc);
-        ret = AVERROR(errno);
-        goto end;
-    }
+    av_assert0(cd != (iconv_t)-1);
 
     inb = inpkt->data;
     inl = inpkt->size;