Fix an issue in ncp_spinel.cpp related to a dummy va_list variable …

…being uninitialized. It causes problems with certain architectures and compiler flags, and initializing it using va_start doesn't work since it's not in a variadic function.

src/ncp/ncp_spinel.cpp

@@ -69,9 +69,10 @@ void NcpSpinel::GetDeviceRole(GetDeviceRoleHandler aHandler)
 {
     otError      error = OT_ERROR_NONE;
     spinel_tid_t tid   = GetNextTid();
-    va_list      args;
+    va_list      args  = {};
 
     error = mSpinelDriver->SendCommand(SPINEL_CMD_PROP_VALUE_GET, SPINEL_PROP_NET_ROLE, tid, nullptr, args);
+
     if (error != OT_ERROR_NONE)
     {
         FreeTid(tid);