Prevent notice during accessing "cached magnification factor" offset

Sheet View Settings Block should be 8-14 bytes long in BIFF8 Excel 97 according
to the open office file format documentation. However access to byte 10 and 12 is
not possible when record data is malformed, so getUInt2d throws notice.

Author: Jan-Simon Winkelmann

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ### Fixed
 
 - Ensure that the list of shared formulae is maintained when an xlsx file is chunked with readFilter[Issue #169](https://github.com/PHPOffice/PhpSpreadsheet/issues/1669).
+- Fix for notice during accessing "cached magnification factor" offset [#1354](https://github.com/PHPOffice/PhpSpreadsheet/pull/1354)
 
 ## 1.15.0 - 2020-10-11
 

src/PhpSpreadsheet/Reader/Xls.php

@@ -4377,11 +4377,22 @@ private function readWindow2(): void
             // offset: 10; size: 2; cached magnification factor in page break preview (in percent); 0 = Default (60%)
             // offset: 12; size: 2; cached magnification factor in normal view (in percent); 0 = Default (100%)
             // offset: 14; size: 4; not used
-            $zoomscaleInPageBreakPreview = self::getUInt2d($recordData, 10);
+            if (!isset($recordData[10])) {
+                $zoomscaleInPageBreakPreview = 0;
+            } else {
+                $zoomscaleInPageBreakPreview = self::getUInt2d($recordData, 10);
+            }
+
             if ($zoomscaleInPageBreakPreview === 0) {
                 $zoomscaleInPageBreakPreview = 60;
             }
-            $zoomscaleInNormalView = self::getUInt2d($recordData, 12);
+
+            if (!isset($recordData[12])) {
+                $zoomscaleInNormalView = 0;
+            } else {
+                $zoomscaleInNormalView = self::getUInt2d($recordData, 12);
+            }
+
             if ($zoomscaleInNormalView === 0) {
                 $zoomscaleInNormalView = 100;
             }