CVE-2026-107213: Excelize: Nil-pointer dereference in GetSlicers when a worksheet has extLst present but no drawing element

GetSlicers guards only if ws.ExtLst == nil { return } (slicer.go:816-818) and then immediately dereferences ws.Drawing.RID with no nil check on ws.Drawing itself. Drawing and ExtLst are two independently optional child elements of in xl/worksheets/sheetN.xml, populated separately by encoding/xml depending on whether each tag is present. I executed a PoC: took a plain NewFile() workbook (no drawing, no slicer), saved it, then injected a bare <extLst></extLst> immediately before </worksheet> in xl/worksheets/sheet1.xml (no other change, no drawing element present) and called GetSlicers. Result: runtime error: invalid memory address or nil pointer dereference, confirmed at the ws.Drawing.RID line via stack trace.