Avoid crash in StringReference.Value JDWP command

Checks for null or invalid object id. Also checks whether the corresponding
object is a java.lang.String.

Bug: 17492221
Bug: 15005460

(cherry picked from commit 29259fa6b0514866d2d4bf57d58c1557b26abbb7)

Change-Id: I52673bdef6912a4cccf5a6eeecb6e1e817b9dd6b
3 files changed