fix SCardCancel() that was broken in version 1.8.21

Changes in 9a21c82 broke
SCardCancel().
The problem was that SCardCancel() was blocked until
SCardGetStatusChange() had finished. This is stupid because
SCardCancel() is supposed to cancel SCardGetStatusChange().

Thanks to Ran Benita for the bug report
"[Pcsclite-muscle] A recent change causes SCardCancel() to block?"
http://lists.alioth.debian.org/pipermail/pcsclite-muscle/2017-June/000921.html

src/winscard_clnt.c

@@ -3118,15 +3118,17 @@ LONG SCardCancel(SCARDCONTEXT hContext)
 	/*
 	 * Make sure this context has been opened
 	 */
-	currentContextMap = SCardGetAndLockContext(hContext);
+	(void)SCardLockThread();
+	currentContextMap = SCardGetContextTH(hContext);
+
 	if (NULL == currentContextMap)
 	{
+		(void)SCardUnlockThread();
 		rv = SCARD_E_INVALID_HANDLE;
 		goto error;
 	}
-
 	cancellable = currentContextMap->cancellable;
-	(void)pthread_mutex_unlock(&currentContextMap->mMutex);
+	(void)SCardUnlockThread();
 
 	if (! cancellable)
 	{