Add a missing mutex unlock in malloc_init_hard().

If multiple threads race to initialize malloc, the loser(s) busy-wait
until initialization is complete.  Add a missing mutex lock so that the
loser(s) properly release the initialization mutex.  Under some
race conditions, this flaw could have caused one or more threads to
become permanently blocked.

Reported by Terrell Magee.

jemalloc/src/jemalloc.c

@@ -324,6 +324,7 @@ malloc_init_hard(void)
 			CPU_SPINWAIT;
 			malloc_mutex_lock(&init_lock);
 		} while (malloc_initialized == false);
+		malloc_mutex_unlock(&init_lock);
 		return (false);
 	}