[LyX/master] Avoid a copy

[Richard Kimberly Heck]

commit 9e4709e4de29c749673f7c61bc1cfccea657bcda
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Sun May 3 02:52:56 2020 -0400

    Avoid a copy
---
 src/insets/InsetCitation.cpp |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/insets/InsetCitation.cpp b/src/insets/InsetCitation.cpp
index 4d1bb68..497f313 100644
--- a/src/insets/InsetCitation.cpp
+++ b/src/insets/InsetCitation.cpp
@@ -384,7 +384,7 @@ docstring InsetCitation::complexLabel(bool for_xhtml) const
 	// we need to give the other defaults, too, to set it.
 	vector<docstring> keys =
 		getVectorFromString(key, from_ascii(","), false, false);
-	for (auto k : keys) {
+	for (auto const & k : keys) {
 		if (biblist.find(k) == biblist.end()) {
 			setBroken(true);
 			break;