TMF: Fix resource leak when trace indexer is re-created

TMF: Fix resource leak when trace indexer is re-created

That happens when a trace was created with the default constructor, a
default indexer is set and when the trace is initialized, a new indexer
is created again, but the previous one is not disposed and the file
remains opened.

Change-Id: I1cb163130563419c81833fd9d383a3eeb2a4f9ee
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/28221
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Hudson CI