fix: Do not panic on cleaning up failed iterators

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

query/iterator.go

@@ -43,9 +43,23 @@ func (a Iterators) Stats() IteratorStats {
 }
 
 // Close closes all iterators.
-func (a Iterators) Close() error {
-	for _, itr := range a {
-		itr.Close()
+// 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
+	if a != nil {
+		for _, itr := range a {
+			if itr != nil {
+				if e := itr.Close(); e != nil && err == nil {
+					err = e
+				}
+			}
+		}
+		return err
 	}
 	return nil
 }