Fix: avoid double-free in build_index_from_idx_file

If the validation at the end of build_index_from_idx_file fails, the
index_entry variable will still point to the last processed index entry.
That same entry will also have been added to the index->entries array.

In the error path, we free index_entry and the index object, which frees
that index entry twice.

Fix it by clearing index_entry after adding the entry to the index
object (the ownership is conceptually transferred).

I don't add a test with this patch, because the file that triggers this
bug now hits a bug further in the processing.  That file will be added
in the testsuite when it will no longer make babeltrace crash.

Change-Id: I091785895541105273c5d07d49f35628c2682e30
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2211
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>