v8: fix debug builds on Windows

Adds missing return which fixes debug builds on Windows

Fixes: nodejs#13392
Ref: https://codereview.chromium.org/2929993003/
PR-URL: nodejs#13634
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

deps/v8/src/api.cc

@@ -437,7 +437,10 @@ void V8::SetSnapshotDataBlob(StartupData* snapshot_blob) {
   i::V8::SetSnapshotBlob(snapshot_blob);
 }
 
-void* v8::ArrayBuffer::Allocator::Reserve(size_t length) { UNIMPLEMENTED(); }
+void* v8::ArrayBuffer::Allocator::Reserve(size_t length) {
+  UNIMPLEMENTED();
+  return nullptr;
+}
 
 void v8::ArrayBuffer::Allocator::Free(void* data, size_t length,
                                       AllocationMode mode) {