[AssetExtractor] Check for null before trying to print error msg

Author: ViliusSutkus89

pdf2htmlEX/src/main/java/com/viliussutkus89/android/pdf2htmlex/AssetExtractor.java

@@ -68,7 +68,10 @@ private static Boolean copyFile(InputStream input, OutputStream output) {
             in.close();
             input.close();
         } catch (IOException e) {
-            Log.e(TAG, e.getMessage());
+            String msg = e.getMessage();
+            if (null != msg) {
+                Log.e(TAG, msg);
+            }
             return false;
         }
         return true;