handle missing end bytes for cmap code

when the byte input does not contain a code of the full code length we return early.

src/UglyToad.PdfPig/PdfFonts/Cmap/CMap.cs

@@ -141,6 +141,11 @@
 
             for (int i = 1; i < minCodeLength; i++)
             {
+                if (bytes.IsAtEnd())
+                {
+                    break;
+                }
+
                 result[i] = ReadByte(bytes);
             }