[driver] fixing possible app/driver/nfsd race

entry->filename points to something the app address space. if app was
started and the ctrl-ed c, then when nfsd picks up the upcall it should
check if the pointer is still valid

sys/nfs41_driver.c

@@ -558,10 +558,13 @@ NTSTATUS marshal_nfs41_header(
     tmp += sizeof(HANDLE);
 
 #ifdef DEBUG_MARSHAL_HEADER
+    if (MmIsAddressValid(entry->filename))
         DbgP("[upcall header] xid=%lld opcode=%s filename=%wZ version=%d "
             "session=0x%x open_state=0x%x\n", entry->xid, 
             opcode2string(entry->opcode), entry->filename,
             entry->version, entry->session, entry->open_state);
+    else
+        status = STATUS_INTERNAL_ERROR;
 #endif
 out:
     return status;