Cleanup minor pointer arithmetic issue (#4495)

Author: jasnell

src/workerd/jsg/buffersource-test.c++

@@ -55,11 +55,29 @@ struct BufferSourceContext: public jsg::Object, public jsg::ContextGlobal {
     return BufferSource(js, BackingStore::alloc<v8::ArrayBuffer>(js, 3));
   }
 
+  bool doTest(jsg::Lock& js, jsg::BufferSource buf) {
+    buf.asArrayPtr()[0] = 1;
+    buf.asArrayPtr()[1] = 2;
+    buf.asArrayPtr()[2] = 3;
+    buf.asArrayPtr()[3] = 4;
+    buf.asArrayPtr()[4] = 5;
+    buf.asArrayPtr()[5] = 6;
+    buf.asArrayPtr()[6] = 7;
+    buf.asArrayPtr()[7] = 8;
+
+    auto ptr = buf.asArrayPtr<uint32_t>();
+    KJ_ASSERT(ptr.size() == 2);
+    KJ_ASSERT(ptr[0] == 0x04030201);
+    KJ_ASSERT(ptr[1] == 0x08070605);
+    return true;
+  }
+
   JSG_RESOURCE_TYPE(BufferSourceContext) {
     JSG_METHOD(takeBufferSource);
     JSG_METHOD(takeUint8Array);
     JSG_METHOD(makeBufferSource);
     JSG_METHOD(makeArrayBuffer);
+    JSG_METHOD(doTest);
   }
 };
 JSG_DECLARE_ISOLATE_TYPE(BufferSourceIsolate, BufferSourceContext);
@@ -99,6 +117,8 @@ KJ_TEST("BufferSource works") {
                "u8.byteLength === 0 && u2.byteLength === 1 && u2 instanceof Uint8Array && "
                "u2.buffer.byteLength === 4 && u2.byteOffset === 1 && u8 !== u2",
       "boolean", "true");
+
+  e.expectEval("const buf = new Uint8Array(12); doTest(buf.subarray(4))", "boolean", "true");
 }
 
 }  // namespace

src/workerd/jsg/buffersource.h

@@ -130,8 +130,8 @@ class BackingStore {
   inline kj::ArrayPtr<T> asArrayPtr() KJ_LIFETIMEBOUND {
     KJ_ASSERT(backingStore != nullptr, "Invalid access after move.");
     KJ_ASSERT(byteLength % sizeof(T) == 0);
-    return kj::ArrayPtr<T>(
-        static_cast<T*>(backingStore->Data()) + byteOffset, byteLength / sizeof(T));
+    kj::byte* data = static_cast<kj::byte*>(backingStore->Data());
+    return kj::ArrayPtr<T>(reinterpret_cast<T*>(data + byteOffset), byteLength / sizeof(T));
   }
 
   template <typename T = kj::byte>