Check if fence is nullptr in PostHandleQueueSignal

PostHandleQueueSignal assumed that fence was not null because QueueSignal
generates an error if it is. The errors are not surfaced immediately so
this additional check is needed before doing the post-handler.

Bug: chromium:914808
Change-Id: I2a99f5229712d49d3c9a2d1f3f2dd1009247a24c
Reviewed-on: https://dawn-review.googlesource.com/c/3280
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

generator/templates/dawn_wire/WireServer.cpp

@@ -509,6 +509,9 @@ namespace dawn_wire {
                 {% set custom_post_handler_commands = ["QueueSignal"] %}
 
                 bool PostHandleQueueSignal(const QueueSignalCmd& cmd) {
+                    if (cmd.fence == nullptr) {
+                        return false;
+                    }
                     ObjectId fenceId = mFenceIdTable.Get(cmd.fence);
                     ASSERT(fenceId != 0);
                     auto* fence = mKnownFence.Get(fenceId);

src/dawn_native/Fence.cpp

@@ -18,7 +18,6 @@
 #include "dawn_native/Device.h"
 #include "dawn_native/ValidationUtils_autogen.h"
 
-#include <cstdio>
 #include <utility>
 
 namespace dawn_native {