Fix crash issue

Changes are from PhilJay#5376

Author: hannesa2

MPChartLib/src/main/java/com/github/mikephil/charting/charts/Chart.java

@@ -751,7 +751,7 @@ protected void drawMarkers(Canvas canvas) {
 			Entry e = mData.getEntryForHighlight(highlight);
 
 			// make sure entry not null before using it
-			if (e == null) {
+			if (e == null || set == null) {
 				continue;
 			}
 

MPChartLib/src/main/java/com/github/mikephil/charting/charts/CombinedChart.java

@@ -240,7 +240,7 @@ protected void drawMarkers(Canvas canvas) {
 			IDataSet set = mData.getDataSetByHighlight(highlight);
 
 			Entry e = mData.getEntryForHighlight(highlight);
-			if (e == null) {
+			if (e == null || set == null) {
 				continue;
 			}