Fix 'implicit conversion from size_t to int changes value' clang warning

Fix 'implicit conversion from size_t to int changes value' clang warning
(fix of commit 9468065)

* misc.c (GC_get_size_map_at): Replace (size_t)(signed_word)-1 to
GC_SIZE_MAX.
* tests/test.c [!GC_GET_HEAP_USAGE_NOT_NEEDED] (check_heap_stats):
Replace (size_t)(GC_signed_word)-1 to -1.