memory_pool: store allocated size before pointer address

Before:
[boisvert@bigmem biosal]$ grep memory_allocate log|awk '{print $10}'|sort|wc -l
60478

After:
[boisvert@bigmem biosal]$ grep memory_allocate log|awk '{print $10}'|sort|wc -l
39965

Link: https://github.com/GeneAssembly/biosal/issues/819
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>