Excelize: Unchecked pivot-cache field index in extractPivotTableFields causes unrecoverable panic

QFinch assessment

extractPivotTableFields builds order := pc.getPivotCacheFieldsName() from xl/pivotCache/pivotCacheDefinitionN.xml's <cacheFields> list, then indexes it with values taken from a separately parsed xl/pivotTables/pivotTableN.xml part with zero cross-validation: order[field.Fld] where field.Fld is a raw attacker-controlled int from the <dataField fld="N"> attribute, and order[fieldIdx] where fieldIdx is a loop index over pt.PivotFields.PivotField that need not match the cache's field count. I executed two independent PoCs against the current HEAD: (1) trimmed the pivot cache's <cacheFields> to count=0 while leaving the pivot table's 2 pivotFields untouched -> runtime error: index out of range [0] with length 0 at pivotTable.go:1431; (2) left the 2 cache fields completely intact and only changed one attribute, fld="1" -> fld="99999", in xl/pivotTables/pivotTable1.xml -> runtime error: index out of range [99999] with length 2 at pivotTable.go:1443. Both stack traces confirmed via non-recovered go test runs: extractPivotTableFields -> getPivotTable -> GetPivotTables.