Prefer not_reached() over assert(false) where appropriate.
Consistently use malloc_mutex_prefork().

Consistently use malloc_mutex_prefork() instead of malloc_mutex_lock()
in all prefork functions.
Fix a data race for large allocation stats counters.

Reported by Pat Lynch.
Avoid (x < 0) comparison for unsigned x.

Avoid (min < 0) comparison for unsigned min in malloc_conf_init().  This
bug had no practical consequences.

Reported by Pat Lynch.
Add a missing mutex unlock in malloc_init_hard() error path.

Add a missing mutex unlock in a malloc_init_hard() error path (failed
mutex initialization).  In practice this bug was very unlikely to ever
trigger, but if it did, application deadlock would likely result.

Reported by Pat Lynch.
Fix a file descriptor leak in a prof_dump_maps() error path.

Reported by Pat Lynch.