fix: guard dlt_log_handle close+reset under dlt_mutex (CWE-362, MISRA R1.3)

[aki1770-del]

Fixes #253.

Problem

dlt_user.dlt_log_handle is closed and reset without holding dlt_mutex in the DLT_RETURN_PIPE_ERROR case of dlt_user_log_send_log_v2() (src/lib/dlt_user.c):

// Before — no mutex held:
close(dlt_user.dlt_log_handle);
dlt_user.dlt_log_handle = -1;

Concurrent threads calling DLT_LOG_X() macros may read dlt_log_handle between the close() and the = -1 assignment (TOCTOU window), or write to a file descriptor that has already been closed and reallocated to an unrelated resource (fd-reuse hazard).

Safety classification:

• CWE-362: Race Condition (TOCTOU on dlt_log_handle check + close)
• CWE-416: fd-reuse hazard (closed fd may be reallocated to an unrelated resource)
• MISRA C:2012 Rule 1.3: Undefined behavior — C11 §5.1.2.4 defines a data race on a non-atomic object as undefined behavior
• AUTOSAR SWS_DLT_01000: freedom from interference between software components using the DLT API concurrently

Fix

Wrap the close+reset block under dlt_mutex_lock()/dlt_mutex_unlock():

// After — atomic close+reset:
dlt_mutex_lock();
close(dlt_user.dlt_log_handle);
dlt_user.dlt_log_handle = -1;
dlt_mutex_unlock();

dlt_mutex is a PTHREAD_MUTEX_RECURSIVE mutex — no deadlock risk from nested acquisition. The v1 send path (dlt_user_log_send_log()) already acquires dlt_mutex at function entry; this patch closes the equivalent gap introduced in the v2 path.

Secondary fix

Removes an orphaned dlt_mutex_unlock() in dlt_free() that was left without a matching dlt_mutex_lock(), immediately after the dlt_receiver_free() block. This unbalanced unlock would corrupt the mutex state on any platform that detects unlock-without-lock.

Continuity

• dlt_user_init was made thread-safe in a prior patch (see ReleaseNotes.md: "dlt_user: Make dlt_init thread safe"). This PR closes the corresponding gap in the deinitialization and DLTv2 send paths.
• Active MISRA compliance work is noted in ReleaseNotes.md ("DLT MISRA conform changes"). This patch closes the most significant remaining MISRA-relevant gap in dlt_user.c (Rule 1.3 — undefined behavior via data race).

Change scope

src/lib/dlt_user.c only. +2 lines, −3 lines. No new dependencies. No new files. No behavior change on the non-error path.