db: adjust Iterator.NextPrefix to error if in error state

Revert the changes to Iterator.nextPrefix in bfd3ee2. Instead, adjust
Iterator.NextPrefix to error if the iterator is already an error state.  This
mirrors the behavior of Iterator.Next.

Fix #3597.

iterator.go

@@ -1720,6 +1720,9 @@ func (i *Iterator) NextPrefix() bool {
 		i.iterValidityState = IterExhausted
 		return false
 	}
+	if i.Error() != nil {
+		return false
+	}
 	return i.nextPrefix() == IterValid
 }
 
@@ -1740,11 +1743,6 @@ func (i *Iterator) nextPrefix() IterValidityState {
 		// key.
 		i.rangeKey.updated = i.rangeKey.hasRangeKey && !i.Valid() && i.opts.rangeKeys()
 	}
-	// NextPrefix from an exhausted position is undefined. We keep the exhausted
-	// position, which provides determinism for the metamorphic tests.
-	if i.iterValidityState == IterExhausted {
-		return i.iterValidityState
-	}
 
 	// Although NextPrefix documents that behavior at IterAtLimit is undefined,
 	// this function handles these cases as a simple prefix-agnostic Next. This