PDFBOX-5660: avoid ArrayIndexOutOfBoundsException, as suggested by Ze Sheng; closes #411

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1931613 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

fontbox/src/main/java/org/apache/fontbox/cmap/CMapParser.java

@@ -797,6 +797,10 @@ private static boolean isDelimiter(int aByte)
 
     private static boolean increment(byte[] data, int position, boolean useStrictMode)
     {
+        if (position < 0)
+        {
+            return false;
+        }
         if (position > 0 && (data[position] & 0xFF) == 255)
         {
             // PDFBOX-4661: avoid overflow of the last byte, all following values are undefined