Nil pointer dereference when getting cell formula

[lc-jordanfelix]

Description
When trying to get a cell formula, if the cell has a nil formula, you can get a nil pointer dereference error.

In GetCellFormula() (cell.go line 243+)

for k := range xlsx.SheetData.Row {
	if xlsx.SheetData.Row[k].R == row {
		for i := range xlsx.SheetData.Row[k].C {
			if axis == xlsx.SheetData.Row[k].C[i].R {
				if xlsx.SheetData.Row[k].C[i].F.T == STCellFormulaTypeShared {
					return getSharedForumula(xlsx, xlsx.SheetData.Row[k].C[i].F.Si)
				}
				if xlsx.SheetData.Row[k].C[i].F != nil {
					return xlsx.SheetData.Row[k].C[i].F.Content
				}
			}
		}
	}
}

->

for k := range xlsx.SheetData.Row {
	if xlsx.SheetData.Row[k].R == row {
		for i := range xlsx.SheetData.Row[k].C {
			if axis == xlsx.SheetData.Row[k].C[i].R {
				if xlsx.SheetData.Row[k].C[i].F != nil {
					if xlsx.SheetData.Row[k].C[i].F.T == STCellFormulaTypeShared {
						return getSharedForumula(xlsx, xlsx.SheetData.Row[k].C[i].F.Si)
					}
					return xlsx.SheetData.Row[k].C[i].F.Content
				}
			}
		}
	}
}

Steps to reproduce the issue:

1. Have a cell with a nil formula
2. Try running GetCellFormula on above cell

Describe the results you received:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x1175ce8]

Describe the results you expected:
Empty string
Output of go version:

go version go1.11.2 darwin/amd64

Excelize version or commit ID:

github.com/360EntSecGroup-Skylar/excelize v1.4.0

Environment details (OS, Microsoft Excel™ version, physical, etc.):
macOS Mojave (Version 10.14) Terminal