Fix case where mergeComplexNumberFormatMasks would get stuck in endle…

…ss-loop if $numbers had many decimals

src/PhpSpreadsheet/Style/NumberFormat.php

@@ -562,7 +562,7 @@ private static function mergeComplexNumberFormatMasks($numbers, $masks)
         $postDecimalMasks = [];
 
         do {
-            $tempMask = array_pop($masks);
+            if (($tempMask = array_pop($masks)) === null) break;
             $postDecimalMasks[] = $tempMask;
             $decimalCount -= strlen($tempMask);
         } while ($decimalCount > 0);