utrace: fix memory leak

utrace: fix memory leak

Fixes following memory leak:

  14 bytes in 1 blocks are definitely lost in loss record 1 of 5
     at 0x4079514: malloc (vg_replace_malloc.c:309)
     by 0x4049A04: vasprintf (vasprintf.c:13)
     by 0x4046354: asprintf (asprintf.c:10)
     by 0x80491A9: main (in /root/utrace)

  134 bytes in 1 blocks are definitely lost in loss record 3 of 5
     at 0x4079514: malloc (vg_replace_malloc.c:309)
     by 0x4049A04: vasprintf (vasprintf.c:13)
     by 0x4046354: asprintf (asprintf.c:10)
     by 0x8049208: main (in /root/utrace)

Signed-off-by: Junnan Xu <junnanx.xu@gmail.com>
Reviewed-by: Rui Salvaterra <rsalvaterra@gmail.com>