JSLock release should only modify the AtomicStringTable if it modified in acquire

JSLock release should only modify the AtomicStringTable if it modified in acquire
https://bugs.webkit.org/show_bug.cgi?id=135143

Reviewed by Darin Adler.

* runtime/JSLock.cpp:
(JSC::JSLock::JSLock):
Initialize the member variable to nullptr.

(JSC::JSLock::willDestroyVM):
Update style to use nullptr instead of 0.

(JSC::JSLock::willReleaseLock):
We should only reset the thread data's atomic string table if
didAcquireLock changed it. m_entryAtomicStringTable will have
been set by didAcquireLock if it changed, or nullptr if it didn't.
This way we are sure we are balanced, regardless of m_vm changes.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171558 268f45cc-cd09-0410-ab3c-d52691b4dbfc