hwdb: bounds-check trie offsets against the mmap before dereferencing

hwdb: bounds-check trie offsets against the mmap before dereferencing

Fixes a family of OOB reads in the trie walker where attacker-controlled
offsets are loaded from a (possibly hostile) hwdb.bin and added to
hwdb->map without first checking that the resulting pointer stays inside
the mapping. Crash sites:

  - trie_fnmatch_f               sd-hwdb.c:187   (#42340)
  - trie_search_f                sd-hwdb.c:233   (#42341)
  - trie_children_cmp_f /bsearch sd-hwdb.c:94    (#42342)
  - hwdb_add_property            sd-hwdb.c:121   (#42343)
  - trie_node_from_off (UBSAN)   sd-hwdb.c:83
  - trie_fnmatch_f stack overflow on cyclic children (CIFuzz finding)

Commit b45a897edc ("hwdb: reject out-of-bounds fnmatch prefixes")
plugged the first site only at the prefix_off-content level, but the
crash on the read of node->prefix_off itself still happens when node is
already OOB. This is the structural fix: a single hwdb_at() helper
validates that [off, off + size) lies inside the mapping, and
trie_node_from_off() / trie_string() return NULL on OOB; every caller
treats the result as nullable and surfaces -EBADMSG.

trie_fnmatch_f() additionally caps its recursion depth at 2048; without
the cap a corrupt trie whose child offsets form a cycle (or just a deep
linear chain) drives the parser into stack-overflow rather than
returning -EBADMSG (caught by CIFuzz address sanitizer on this branch).

Additions:

  - A fuzz-hwdb libFuzzer harness that drives sd_hwdb_new_from_path,
    sd_hwdb_get, sd_hwdb_seek, and sd_hwdb_enumerate on attacker bytes.
  - Five hand-crafted unit tests in test-sd-hwdb.c (one per crash
    bucket, plus the cyclic-trie recursion case) that build a malformed
    in-memory hwdb.bin and assert -EBADMSG rather than SIGSEGV /
    stack-overflow.
  - Regression corpus files under test/fuzz/fuzz-hwdb/ pinning each
    fixed bucket, including the CIFuzz stack-overflow reproducer.

Closes #42340.
Closes #42341.
Closes #42342.
Closes #42343.

Reported-by: AI-assisted libFuzzer campaign
Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>