[ncp] fix a compilation issue in ncp_spinel.cpp

Fix an issue in `ncp_spinel.cpp` related to a dummy va_list variable being uninitialized.

src/ncp/ncp_spinel.cpp

@@ -190,7 +190,7 @@ void NcpSpinel::ThreadErasePersistentInfo(AsyncTaskPtr aAsyncTask)
 {
     otError      error = OT_ERROR_NONE;
     spinel_tid_t tid   = GetNextTid();
-    va_list      args;
+    va_list      args  = {};
 
     VerifyOrExit(mThreadErasePersistentInfoTask == nullptr, error = OT_ERROR_BUSY);