Replace assert with AVERROR when recode_subtitle called w/o ICONV.

recode_subtitle() is called implicitly by avformat_find_stream_info().
As such, clients which disable ICONV always crash if a file contains
subtitles; even if they don't care about them.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -2553,7 +2553,7 @@ end:
         iconv_close(cd);
     return ret;
 #else
-    av_assert0(!"requesting subtitles recoding without iconv");
+    return AVERROR(EINVAL);
 #endif
 }