Eliminate 64-bit compiler warnings in atomic_ops_malloc

Eliminate 64-bit compiler warnings in atomic_ops_malloc

* src/atomic_ops_malloc.c (add_chunk_as): Cast shifted value to size_t
(the type of assigned variable, at least to eliminate compiler warning
for 64-bit targets).
* src/atomic_ops_malloc.c (AO_free): Cast (*base) to int (the type of
assigned variable) to eliminate MS VC compiler warning.