[api] Fix compilation error for `UNIMPLEMENTED()` method

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index f009cfe6836b1392039c290f63a41bff220da252..ab43081db23baa8dccda8b24124f5b2d5b60f9c4 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -453,7 +453,10 @@ 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) {