Fix: don't free unallocated index

When opening a mmap trace, the index is not created so does not need to
be freed, just add a check before freeing it.
For clarity, this patch also allocates the packet_real_index just after
the packet_cycles_index instead of waiting for the convert timestamp
function.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>