Fix GC_finalized_malloc for out-of-memory case (and for missing

Fix GC_finalized_malloc for out-of-memory case (and for missing
'const' qualifier)

* fnlz_mlc.c (GC_finalized_disclaim): Add 'const' qualifier for "fc"
pointer.
* fnlz_mlc.c (GC_core_finalized_malloc, GC_finalized_malloc): Cast
"op" to 'const' pointer on "fclos" storing.
* fnlz_mlc.c (GC_finalized_malloc): Use GC_get_oom_fn() instead of
locked read of GC_oom_fn.
* fnlz_mlc.c (GC_core_finalized_malloc): Test GC_generic_malloc result
for NULL (return NULL in this case).
* fnlz_mlc.c (GC_core_finalized_malloc): Add assertion on GC_size
returned value (at least just to outline that it might be bigger than
"lb" value).