Be compatible with dalvikvm on calls to env->NewString(NULL, 0);

The previous behavior was causing libcore.java.util.OldScannerTest to
fail with NullPointerExceptions.

Change-Id: I8c9b4e496e6d17a0ad8be10888daa52ac8d178b7
diff --git a/src/object.cc b/src/object.cc
index 27cbb41..49ba5a3 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -1377,6 +1377,7 @@
 String* String::AllocFromUtf16(int32_t utf16_length,
                                const uint16_t* utf16_data_in,
                                int32_t hash_code) {
+  CHECK(utf16_data_in != NULL || utf16_length == 0);
   String* string = Alloc(GetJavaLangString(), utf16_length);
   if (string == NULL) {
     return NULL;