[LyX/master] Avoid a copy.

[Richard Kimberly Heck]

commit 4a81983edd08735fb206b74f2e29bbbd29cb2db0
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Sat Feb 29 05:55:20 2020 -0500

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

diff --git a/src/BiblioInfo.cpp b/src/BiblioInfo.cpp
index 50d8abf..2161bc6 100644
--- a/src/BiblioInfo.cpp
+++ b/src/BiblioInfo.cpp
@@ -1449,7 +1449,7 @@ void BiblioInfo::collectCitedEntries(Buffer const & buf)
 	// FIXME We may want to collect these differently, in the first case,
 	// so that we might have them in order of appearance.
 	set<docstring> citekeys;
-	Toc const toc = *buf.tocBackend().toc("citation");
+	Toc const & toc = *buf.tocBackend().toc("citation");
 	for (auto const & t : toc) {
 		if (t.str().empty())
 			continue;