refactor: use NonNull to deal with pam_getenvlist

[japaric]

I had trouble reading the original version and thought that the curr_str passed into CStr::from_ptr was not checked for null-ness but that was done in while condition -- the not very visible dereferencing operator there through me off.

The new version should hopefully make it clearer that a NonNull pointer is being passed into CStr::from_ptr. (Of course, it'd be even better if CStr::from_ptr signature was fn(NonNull<u8>) but we can't change the libstd API.) It also makes the dereferencing more explicit using the read method.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@6d40763). Click here to learn what that means.
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #758   +/-   ##
=======================================
  Coverage        ?   55.04%           
=======================================
  Files           ?       71           
  Lines           ?     9765           
  Branches        ?        0           
=======================================
  Hits            ?     5375           
  Misses          ?     4390           
  Partials        ?        0           

Files Changed	Coverage Δ
src/pam/mod.rs	0.00% <0.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

Number of dependencies and binary size impact report

Metric	main	PR #758	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	1023.2 KiB	1023.5 KiB	-
Text size	594.9 KiB	594.9 KiB	-

Dependencies diff

 └─ sudo-rs [v0.2.0]
    ├─ glob [v0.3.1]
    ├─ libc [v0.2.147]
    └─ log [v0.4.20]