Prevent zero from being passed to array_chunk() (#686)

Passing a zero (0) value to the `array_chunk()` function causes an error, and in rare cases, a PDF XRef object may be added to a document with an "empty" `/W [0 0 0]` command. This would cause the `$rowlen` variable to be set to zero and cause an error.

Add a simple check to return an empty array in this case.

src/Smalot/PdfParser/RawData/RawDataParser.php

@@ -408,10 +408,15 @@ protected function decodeXrefStream(string $pdfData, int $startxref, array $xref
             } else {
                 // number of bytes in a row
                 $rowlen = array_sum($wb);
-                // convert the stream into an array of integers
-                $sdata = unpack('C*', $xrefcrs[1][3][0]);
-                // split the rows
-                $ddata = array_chunk($sdata, $rowlen);
+                if (0 < $rowlen) {
+                    // convert the stream into an array of integers
+                    $sdata = unpack('C*', $xrefcrs[1][3][0]);
+                    // split the rows
+                    $ddata = array_chunk($sdata, $rowlen);
+                } else {
+                    // if the row length is zero, $ddata should be an empty array as well
+                    $ddata = [];
+                }
             }
 
             $sdata = [];