Using `calloc` function with a count of zero elements is implementation
defined and it may cause allocation errors on some platforms (ref #3277).
This commit fixes `jv_mem_calloc` callers to avoid undefined behavior.
In previous code nomem_handler in jv_alloc.c is called only once
and therefore the structure is not initialized for second and
following threads.
This leads to segmentation fault in multi-threading environment.
This patch moves initialization of nomem_handler out of the
pthread_once call.
itchyny