datapath-windows: Add assert to ethHdr in OvsActionMplsPush

`ethHdr` cannot be NULL because we did a partial copy before it.

Add an assert to keep the static analysis happy.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Shashank Ram <shashank08@gmail.com>

datapath-windows/ovsext/Actions.c

@@ -1237,6 +1237,7 @@ OvsActionMplsPush(OvsForwardingContext *ovsFwdCtx,
     ASSERT(mdlLen >= MPLS_HLEN);
 
     ethHdr = (EthHdr *)(bufferStart + curMdlOffset);
+    ASSERT(ethHdr);
     RtlMoveMemory(ethHdr, (UINT8*)ethHdr + MPLS_HLEN, sizeof(*ethHdr));
     ethHdr->Type = mpls->mpls_ethertype;