environment: use 'repo->initialized' for repo_protect_hfs() and repo_protect_ntfs()

environment: use 'repo->initialized' for repo_protect_hfs() and repo_protect_ntfs()

To match how we refrain from calling repo_config_values() on an
uninitialized instance of a repository object in other two topics
that deal with ignore_case and trust_executable_bit, check the
repo->initialized bit instead of the repo->gitdir member.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Mentored-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>