when calculating the score, check the assignable from too, and return -1 if the check doesn't work.

jnius/jnius_utils.pxi

@@ -169,8 +169,11 @@ cdef int calculate_score(sign_args, args) except *:
                 if jc.__javaclass__ == r:
                     score += 10
                 else:
-                    # can be dangerous
-                    score += 1
+                    try:
+                        check_assignable_from(jc, r)
+                    except:
+                        return -1
+                    score += 5
                 continue
 
             # always accept unknow object, but can be dangerous too.