libavformat/xwma: fix overflow in seek position

Fixes: signed integer overflow

Found-by: Marius Momeu <marius.momeu@berkeley.edu>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2d4ec46345)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/xwma.c

@@ -267,7 +267,7 @@ static int xwma_read_header(AVFormatContext *s)
              * an offset / timestamp pair.
              */
             av_add_index_entry(st,
-                               cur_pos + (i+1) * st->codecpar->block_align, /* pos */
+                               cur_pos + (i+1LL) * st->codecpar->block_align, /* pos */
                                dpds_table[i] / bytes_per_sample,            /* timestamp */
                                st->codecpar->block_align,                   /* size */
                                0,                                           /* duration */