[vm, service] Fix out-of-bounds access when processing requestHeapSnapshot's arguments.

As detected by the ASAN bots.

Change-Id: Ia10ad883afe886d573f0fa32c2096f9e0ac07083
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113757
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 7981794..d212875 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -4064,6 +4064,7 @@
 
 static const MethodParameter* request_heap_snapshot_params[] = {
     RUNNABLE_ISOLATE_PARAMETER,
+    NULL,
 };
 
 static bool RequestHeapSnapshot(Thread* thread, JSONStream* js) {