BUG/MINOR: debug/pools: properly handle out-of-memory when building with DEBUG_UAF

BUG/MINOR: debug/pools: properly handle out-of-memory when building with DEBUG_UAF

Commit 158fa75 ("MINOR: pools: implement DEBUG_UAF to detect use after free")
implemented pool use-after-free detection, but the mmap() return value isn't
properly checked, preventing the call to pool_alloc_area() from returning
NULL. So on out-of-memory a mangled pointer is returned, causing a crash on
the pool_alloc() site instead of forcing a GC. It doesn't affect regular
operations however, just complicates complex bug investigations.

This fix should be backported to 1.8 and to 1.7.