[PATCH v3 1/4] environment.c: fix potential segfault by get_git_common_dir()

setup_git_env() must be called before this function to initialize git_common_dir so that it returns a non NULL string. And it must return a non NULL string or segfault can happen because all callers expect so.

Normally if somebody has called get_git_dir(), or set_git_dir() then setup_git_env() is already called. But if you do setup_git_directory() at top dir (which skips set_git_dir) and never call get_git_dir, you'll get NULL here.

test-ref-store.c will hit this problem because it's very lightweight,
just enough initialization to exercise refs code, and get_git_dir() will
never be called until get_worktrees() is, which uses get_git_common_dir().
---
 environment.c | 2 ++
 1 file changed, 2 insertions(+)