Drowning in Warning Messages with Corrupt Xls File

Fix #4647. Xls file is corrupt - PhpSpreadsheet tries to extract a substring using array notation, but the index is out of bounds. Php treats this as a warning situation, so continues to process, leading to an onslaught of warning messages. We could change to use the `substr` function rather than array notation, but that seems inappropriate - it would be better to throw an exception and have the user fix the file. In the file posted with the issue, opening it with Excel, and responding yes when it asks if it's okay to clean up the corruption, yields a usable file. Unfortunately, that file weighs in at 28MB, much too large for our test suite. So, no new unit tests accompany this change, but it has been tested.

Tests are added to `getUint2d`, which seems to be the source of the problem in the sample file, and, for good measure, `getInt2d` and `getInt4d`. There may be other sources of similar corruption, but we'll stick with what's in front of our nose.

Author: oleibman

src/PhpSpreadsheet/Reader/XlsBase.php

@@ -368,11 +368,20 @@ protected function decodeCodepage(string $string): string
         return StringHelper::convertEncoding($string, 'UTF-8', $this->codepage);
     }
 
+    protected static function confirmPos(string $data, int $pos): void
+    {
+        if ($pos >= strlen($data)) {
+            throw new PhpSpreadsheetException('File appears to be corrupt'); //* @codeCoverageIgnore
+        }
+    }
+
     /**
      * Read 16-bit unsigned integer.
      */
     public static function getUInt2d(string $data, int $pos): int
     {
+        self::confirmPos($data, $pos + 1);
+
         return ord($data[$pos]) | (ord($data[$pos + 1]) << 8);
     }
 
@@ -381,6 +390,8 @@ public static function getUInt2d(string $data, int $pos): int
      */
     public static function getInt2d(string $data, int $pos): int
     {
+        self::confirmPos($data, $pos + 1);
+
         return unpack('s', $data[$pos] . $data[$pos + 1])[1]; // @phpstan-ignore-line
     }
 
@@ -389,6 +400,8 @@ public static function getInt2d(string $data, int $pos): int
      */
     public static function getInt4d(string $data, int $pos): int
     {
+        self::confirmPos($data, $pos + 3);
+
         // FIX: represent numbers correctly on 64-bit system
         // http://sourceforge.net/tracker/index.php?func=detail&aid=1487372&group_id=99160&atid=623334
         // Changed by Andreas Rehm 2006 to ensure correct result of the <<24 block on 32 and 64bit systems