[tools][perfmon] Another fix for nil pointer dereference

When a gerrit RPC errors.

Change-Id: I0227a1ef638c38a0390ce1874090ea31a67fef56
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/183881
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/tools/src/cmd/perfmon/main.go b/tools/src/cmd/perfmon/main.go
index f3c9dd2..925491d 100644
--- a/tools/src/cmd/perfmon/main.go
+++ b/tools/src/cmd/perfmon/main.go
@@ -1013,7 +1013,7 @@
 
 		if err != nil {
 			info := &strings.Builder{}
-			if resp.Body != nil {
+			if resp != nil && resp.Body != nil {
 				body, _ := io.ReadAll(resp.Body)
 				fmt.Fprintln(info, "response:    ", string(body))
 			}