Throw IOException when dict header cannot be read.

Bug: 13106638
Change-Id: Ib48ef0a191a436552c49b6e817dfa13fe1567ae8

java/src/com/android/inputmethod/latin/makedict/Ver4DictDecoder.java

@@ -53,6 +53,10 @@ public class Ver4DictDecoder extends AbstractDictDecoder {
 
     @Override
     public DictionaryHeader readHeader() throws IOException, UnsupportedFormatException {
+        final DictionaryHeader header = mBinaryDictionary.getHeader();
+        if (header == null) {
+            throw new IOException("Cannot read the dictionary header.");
+        }
         return mBinaryDictionary.getHeader();
     }