Added Safe index subscript logic for ChartData

[tringbhagya]

Issue Link 🔗

Crash Issue

• We face a crash when we attempt to highlight the values for the combined chart.

Implementation Details 🚧

• Added safe subscript logic.

Steps to Reproduce:

• Add this code snippet to demo project's CombinedChartViewController class.

func selectHighLights() {
        var highlight: [Highlight] = []
        chartView.data?.dataSets.enumerated().forEach { index, dataSet in
          dataSet.entriesForXValue(0.5).forEach { entry in
            highlight.append(Highlight(x: entry.x, y: entry.y, dataSetIndex: index))
          }
        }
        chartView?.highlightValues(highlight)
    }
      
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        DispatchQueue.main.asyncAfter(deadline: .now() + 2.0, execute: {
          self.selectHighLights()
        })
    }