Make InlinedStructPtr get() return null when null

For consistency with StructPtr and general developer expectations around
APIs named get(), this changes InlinedStructPtr to return a nullptr when
its internal state is NIL, rather than always returning a valid pointer.

Bug: None
Change-Id: Ia3e59af0374362b3a1e53b418b728336b99149c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461750
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Ken Rockot <rockot@google.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815801}

mojo/public/cpp/bindings/struct_ptr.h

@@ -171,7 +171,11 @@ class InlinedStructPtr {
     DCHECK(state_ == VALID);
     return &value_;
   }
-  Struct* get() const { return &value_; }
+  Struct* get() const {
+    if (state_ == NIL)
+      return nullptr;
+    return &value_;
+  }
 
   void Swap(InlinedStructPtr* other) {
     std::swap(value_, other->value_);