tmf: Fix HistogramDataProvider index OOB

For the event statistics histogram view, the HistogramDataProvider
will occasionally fail with an IndexOutOfBoundsException when its
fetchXY method is called before its state-system backed statistics
have been populated with attributes.

Add a check to verify that the y-values provided by the statistics is
is large enough for the x-values.

Signed-off-by: Adam Wendelin <adam.wendelin@ericsson.com>

tmf/org.eclipse.tracecompass.tmf.core/src/org/eclipse/tracecompass/internal/tmf/core/histogram/HistogramDataProvider.java

@@ -126,6 +126,9 @@ public TmfModelResponse<TmfTreeModel<TmfTreeDataModel>> fetchTree(Map<String, Ob
         final ITmfStatistics stats = Objects.requireNonNull(fModule.getStatistics());
         if (selected.contains(fTotalId)) {
             List<Long> values = stats.histogramQuery(filter.getTimesRequested());
+            if (values.size() < n) {
+                return TmfXyResponseFactory.create(TITLE, xValues, Collections.emptyList(), false);
+            }
 
             double[] y = new double[n];
             Arrays.setAll(y, values::get);