BUILD: activity/memprofile: fix a build warning in the posix_memalign handler

BUILD: activity/memprofile: fix a build warning in the posix_memalign handler

A "return NULL" statement was placed for error handling in the
posix_memalign() handler instead of an int errno value, by recent
commit 5ddc8b3ad4 ("MINOR: activity/memprofile: monitor non-portable
calls as well"). Surprisingly the warning only triggered on gcc-4.8.
Let's use ENOMEM instead. No backport needed.