* fix(volume): avoid nil-deref when needle map loader errors
A corrupt .idx whose size is not a multiple of NeedleMapEntrySize sends
the read-only load path into NewSortedFileNeedleMap, which returns
(*SortedFileNeedleMap)(nil) when reverseWalkIndexFile rejects the file.
The multi-value assignment `v.nm, err = NewSortedFileNeedleMap(...)`
parks that typed-nil pointer in the v.nm NeedleMapper interface, so the
subsequent `v.nm != nil` guard still passes — and the post-load
MaxNeedleEnd structural check dispatches through the promoted mapMetric
accessor on a nil receiver, segfaulting the whole volume server at
load time.
Reset v.nm explicitly after every loader failure so the interface is
truly nil, and skip the MaxNeedleEnd check when err is non-nil since
the value would come from a partial walk anyway. NewLevelDbNeedleMap
has the same typed-nil-on-error shape and is fixed the same way.
* fix(volume): close indexFile when needle map load errors
Pre-fix the typed-nil v.nm path either leaked indexFile silently
(SortedFileNeedleMap.Close had a nil-receiver early return) or crashed
(LevelDbNeedleMap.Close had no such guard). With v.nm cleared to nil
on error, the defer cleanup no longer calls Close at all, so the
LoadCompactNeedleMap success-with-error path now also leaks indexFile.
Close indexFile explicitly on each loader error to keep ownership
balanced.
* trim comments
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