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Ensure typed objects descriptor is never located in the first word
(a cherry-pick of commit daa06b3ac from 'master')

Free objects are linked in the collector through the first word in the
object.  This commit prevents placing type descriptor to the first
word of the allocated object by ensuring that the size of typed objects
(requested by a client) is non-zero.

Also, guarantee no nelements*size overflow in GC_make_array_descriptor.

* typd_mlc.c (GC_make_array_descriptor): Add comment that no overflow
guaranteed by caller.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page): If lb is zero, then assume
lb is 1 (byte).
* typd_mlc.c (GC_calloc_explicitly_typed): If lb or n is zero, then
assume that lb*n is 1; check that lb*n does not overflow before
GC_make_array_descriptor() call.

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