Various fixups to unsafe code (#37651)

A collection of fixups of possibly-unsound code and removing some small
useless writes.

Release Notes:

- N/A

crates/util/src/util.rs

@@ -256,6 +256,9 @@ fn load_shell_from_passwd() -> Result<()> {
             &mut result,
         )
     };
+    anyhow::ensure!(!result.is_null(), "passwd entry for uid {} not found", uid);
+
+    // SAFETY: If `getpwuid_r` doesn't error, we have the entry here.
     let entry = unsafe { pwd.assume_init() };
 
     anyhow::ensure!(
@@ -264,7 +267,6 @@ fn load_shell_from_passwd() -> Result<()> {
         uid,
         status
     );
-    anyhow::ensure!(!result.is_null(), "passwd entry for uid {} not found", uid);
     anyhow::ensure!(
         entry.pw_uid == uid,
         "passwd entry has different uid ({}) than getuid ({}) returned",