Bug 1805799 - Make `TSFTextStore::Selection::Length` use `MOZ_ASSERT_IF` instead of `MOZ_ASSERT` to access `Maybe<TS_SELECTION_ACP>` r=m_kato

`ToString(mSelectionForTSF).c_str()` runs `TSFTextStore::Selection::operator<<`
if it's some.  Then, it calls `TSFTextStore::Selection::Length` always.
https://searchfox.org/mozilla-central/rev/17aeb39742eba71e0936ae44a51a54197100166d/widget/windows/TSFTextStore.h#725

However, `TSFTextStore::Selection::mACP` can be nothing if there is no
`Selection` ranges.  Therefore, the `MOZ_ASSERT(mACP->acpEnd >= mACP->acpStart)`
hits `Maybe<T>.isSome()` check in `->` operator.
https://searchfox.org/mozilla-central/rev/17aeb39742eba71e0936ae44a51a54197100166d/widget/windows/TSFTextStore.h#681

Differential Revision: https://phabricator.services.mozilla.com/D164759

widget/windows/TSFTextStore.h

@@ -678,7 +678,7 @@ class TSFTextStore final : public ITextStoreACP,
     }
 
     LONG Length() const {
-      MOZ_ASSERT(mACP->acpEnd >= mACP->acpStart);
+      MOZ_ASSERT_IF(mACP.isSome(), mACP->acpEnd >= mACP->acpStart);
       return mACP.isSome() ? std::abs(mACP->acpEnd - mACP->acpStart) : 0;
     }