vmem: fix segfault in je_malloc() in multi-threaded applications

Due to the race condition in malloc_init_base_pool(), "base pool"
(aka "pool[0]") may be used when not yet initialized.
The problem is observed only for native je_malloc(). vmem_malloc()
and je_pool_malloc() are not affected.
Merge pull request #38 from krzycz/vmem-base_pool_init

vmem: fix segfault in je_malloc() in multi-threaded applications