mempool: silence a GCC false-positive warning when ASan is used
This fixes the false-positive warning 'may be used uninitialized' reported by GCC 11 and 12 (and possibly other versions) when compiling with '--with-sanitizer=address'.
It likely relates to: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109550 or to some of these known bugs in GCC: https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=may%20be%20used%20uninitialized