asfenc: fix leaking asf->index_ptr on error

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 2c8cff2be4)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/asfenc.c

@@ -660,6 +660,7 @@ static int asf_write_header(AVFormatContext *s)
      * It is needed to use asf as a streamable format. */
     if (asf_write_header1(s, 0, DATA_HEADER_SIZE) < 0) {
         //av_free(asf);
+        av_freep(&asf->index_ptr);
         return -1;
     }