Application crashes when you start composing text on Windows

[yukawa]

Description
There is a null pointer access issue in b861ff2, which ends up crashing application immediately after you start composing text.

Steps to reproduce
Steps to reproduce the behavior:

1. Build b861ff2 and install it.
2. Open notepad.exe
3. Enable Mozc then start typing anything.

Expected behavior
No application crash.

Actual behavior
It's crashing here.

mozc/src/win32/tip/tip_edit_session_impl.cc

Lines 455 to 456 in b861ff2

	if (SUCCEEDED(TipCompositionUtil::GetComposition(context, write_cookie)
	.As(&composition))) {

When TipCompositionUtil::GetComposition(context, write_cookie) returns nullptr, subsequent .As(&composition) ends up crashing.

Version or commit-id
b861ff2

Environment

• OS: Windows 11

Additional Information
The fix is on the way. We just need to check the return value, as it used to be.
b861ff2#diff-1a5aa2b35b0e0b109860116f0280b9e3b641f0225f8304c7a3fd70e48a9382c9L454-R456