Workaround 'resource leak' false positives in alloc_MS, bl/envfile_init

Workaround 'resource leak' false positives in alloc_MS, bl/envfile_init

* blacklst.c (GC_bl_init): Add assertion that GC_old_stack_bl and
GC_incomplete_stack_bl are both null prior to their assignment (to the
result of GC_scratch_alloc).
* mark.c (alloc_mark_stack): Replace GC_mark_stack_size!=0 with
GC_mark_stack!=NULL (to ensure no memory leak when GC_mark_stack is
assigned for the first time).
* misc.c [GC_READ_ENV_FILE && (MSWIN32 || MSWINCE || CYGWIN32)]
(GC_envfile_init): Add assertion that GC_envfile_content is null prior
to its assignment.