The post-compaction size validation at line 538 accesses
v.nm.ContentSize() and v.nm.DeletedSize(). If CommitCompact has
concurrently set v.nm to nil, this causes a SIGSEGV. Skip the
validation when v.nm is nil since the actual data copy uses local
needle maps (oldNm/newNm) and is unaffected.
Fixes#8591