Throw illegal argument exception in BinaryDictionary::editDistance

Bug: 7276803
Change-Id: Icd9bdec0a04179a29d00336819d205627f77b71a

java/src/com/android/inputmethod/latin/BinaryDictionary.java

@@ -187,6 +187,9 @@ public final class BinaryDictionary extends Dictionary {
     }
 
     public static int editDistance(String before, String after) {
+        if (before == null || after == null) {
+            throw new IllegalArgumentException();
+        }
         return editDistanceNative(before.toCharArray(), after.toCharArray());
     }