Merge "Nullability check for mntent module" am: e718f8b912

Original change: https://android-review.googlesource.com/c/platform/bionic/+/2482937

Change-Id: I22f0fbe05f62631d59c8f26468bd3ee28cb351af
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>