fix(link-map): guard head conversion from memory to avoid overflow panic

[jrootcache]

Problem

There is a potential panic in LinkMap::head() when the memory-stored head pointer cannot be converted to a u32 (e.g., due to memory corruption or an out-of-range value).

Fix

Make the conversion fallible-safe by replacing the unwrap/expect with .ok(), returning None when the pointer cannot be represented as u32. This prevents panics on edge cases and keeps behavior stable for malformed data.

Why this is safe

The head() method already treats invalid/missing head pointers as None. Using .ok() preserves the existing contract (None for invalid head) and avoids panics in rare edge cases without altering normal operation.