Merge branch 'ps/reftable-alloc-failures-zalloc-fix'

Recent reftable updates mistook a NULL return from a request for
0-byte allocation as OOM and died unnecessarily, which has been
corrected.

* ps/reftable-alloc-failures-zalloc-fix:
  reftable/basics: return NULL on zero-sized allocations
  reftable/stack: fix zero-sized allocation when there are no readers
  reftable/merged: fix zero-sized allocation when there are no readers
  reftable/stack: don't perform auto-compaction with less than two tables

diff --cc reftable/basics.c
Simple merge

diff --cc reftable/merged.c
Simple merge

diff --cc reftable/stack.c
Simple merge