BUG/MINOR: errors: reset diagnostic buffers after thread cleanup

BUG/MINOR: errors: reset diagnostic buffers after thread cleanup

A live worker frees its thread-local diagnostic buffers before EXTRA_OBJS
teardown, whose deinitializers can still emit diagnostics.

ha_free() releases the allocation and clears only the area pointer. It
leaves size, head and data unchanged, while b_is_null() tests size rather
than area. A previously allocated buffer therefore still looks allocated
with a NULL area. Later diagnostics skip lazy allocation and can crash
when accessing the buffer.

Reset both usermsgs_buf and usermsgs_ctx.str to BUF_NULL after freeing
their areas. This clears the stale size and contents metadata so later
diagnostics can allocate fresh buffers. The post-deinit cleanup releases
any buffers reallocated during EXTRA_OBJS teardown.

This could be backported in every maintained versions.