Fix use after free of "filter/expression" attribute

Fix use after free of "filter/expression" attribute

As evidenced by Valgrind:
 Invalid read of size 4
    at 0x402E248: process_traceset (batchAnalysis.c:103)
    by 0x80558E5: lttv_hooks_call (hook.c:272)
    by 0x8053596: main (main.c:219)
  Address 0x4cea1b8 is 88 bytes inside a block of size 256 free'd
    at 0x4024D47: realloc (vg_replace_malloc.c:476)
[...]
    by 0x402E204: process_traceset (batchAnalysis.c:97)
    by 0x80558E5: lttv_hooks_call (hook.c:272)
    by 0x8053596: main (main.c:219)

Signed-off-by: Benjamin Poirier <benjamin.poirier@polymtl.ca>