Fix some compiler warnings in aset.c and generation.c

Fix some compiler warnings in aset.c and generation.c

This fixes a couple of unused variable warnings that could be seen when
compiling with MEMORY_CONTEXT_CHECKING but not USE_ASSERT_CHECKING.
Defining MEMORY_CONTEXT_CHECKING without asserts is a little unusual,
however, we shouldn't be producing any warnings from such a build.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_D-vgLEh7eO47p=73u1jWO78NWf6Qfv1FndY1kG-Q-jA@mail.gmail.com