Remove mark_maybe_object

Remove mark_maybe_object

* src/alloc.c (mark_maybe_object, mark_maybe_objects): Remove.
(mark_objects): New function.
* src/eval.c (mark_specpdl): Use mark_objects instead of
mark_maybe_objects, since the array now has only valid Lisp objects.
* src/lisp.h (SAFE_ALLOCA_LISP_EXTRA): When allocating a large
array, clear it so that it contains only valid Lisp objects.  This
is simpler and safer, and does not hurt performance significantly
on my usual benchmark as the code is executed so rarely.