fix: do not panic on cleaning up failed iterators (#21666) (#23319)

We have seen occasional panics in Iterators.Close()
when cleaning up after failed iterator creation.
This commit checks for nil on any iterator to be
closed, and now returns any errors generated by
that Close().

Closes #19579
Closes #19476

(cherry picked from commit acc4105)

closes #23271

influxql/query/iterator.go

@@ -43,11 +43,22 @@ func (a Iterators) Stats() IteratorStats {
 }
 
 // Close closes all iterators.
-func (a Iterators) Close() error {
+// We are seeing an occasional panic in this function
+// which looks like a nil reference from one
+// itr.Close() call, thus we check for nil elements
+// in the slice a.  This is often called as error
+// clean-up, so the state of the iterators may be
+// unhappy.
+func (a Iterators) Close() (err error) {
+	err = nil
 	for _, itr := range a {
-		itr.Close()
+		if itr != nil {
+			if e := itr.Close(); e != nil && err == nil {
+				err = e
+			}
+		}
 	}
-	return nil
+	return err
 }
 
 // filterNonNil returns a slice of iterators that removes all nil iterators.