Fix crash when using GC_malloc_many() as first allocation call

* mallocx.c (GC_CALL GC_generic_malloc_many): Do not dereference
ok_reclaim_list if it is NULL.
Fix 'op_sz variable unused' warning in GC_core_finalized_malloc
(fix commit 2a3cef3)

* fnlz_mlc.c (GC_core_finalized_malloc): Remove "op_sz" local variable.