storage: fix UnsafeKey() usage of invalid iterator

Previously, CheckSSTConflicts would improperly call UnsafeKey on an exhausted
sstIter. This could result in undefined, inconsistent behavior as UnsafeKey
may point to arbitrary data once the iterator is exhausted.

Informs cockroachdb#94141.
Epic: None
Release note: None

pkg/storage/sst.go

@@ -1077,7 +1077,7 @@ func CheckSSTConflicts(
 				// 2) the ext iterator became invalid
 				// 3) both iterators changed keys and the sst iterator's key is further
 				//    ahead.
-				if extChangedKeys && (!sstChangedKeys || (!extOK && sstOK) || extIter.UnsafeKey().Key.Compare(sstIter.UnsafeKey().Key) < 0) {
+				if sstOK && extChangedKeys && (!sstChangedKeys || !extOK || extIter.UnsafeKey().Key.Compare(sstIter.UnsafeKey().Key) < 0) {
 					extIter.SeekGE(MVCCKey{Key: sstIter.UnsafeKey().Key})
 					extOK, extErr = extIter.Valid()
 				}