tmf: Fix opening symbol config dialog for experiments

tmf: Fix opening symbol config dialog for experiments

An ArrayIndexOutOfBounds exception could be thrown when opening
the symbol mapping configuration dialog in the context of a
multi-trace experiment. Index "-1" would be passed to
updateMessage(int).

When the window is first opened, the selection index is at -1,
simply skip the updateMessage() in this case.

Change-Id: I43b384016ec7dfff1b16b272b34155579bc101cc
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/70314
Reviewed-by: Hudson CI
Reviewed-by: Robert Kiss <robert.kiss@gmail.com>