[UMPNPMGR] Fix uninitialized variable (#5310)

The unhiding and guilty revision that was identified by @stasm by bisecting the
bootability of LiveCD was 0.4.15-dev-6050-g13e6cbc
As that was the symptom: "We could not longer boot livecd". CORE-18986

base/services/umpnpmgr/rpcserver.c

@@ -696,7 +696,7 @@ PNP_ReportLogOn(
     DWORD ProcessId)
 {
     DWORD ReturnValue = CR_FAILURE;
-    HANDLE hProcess;
+    HANDLE hProcess = NULL;
 
     UNREFERENCED_PARAMETER(Admin);