[LyX/master] Use const &.

[Richard Heck]

commit 499fc04c9a417cb91ddc97a2110ac59882fd4909
Author: Richard Heck <rgh...@lyx.org>
Date:   Wed Jun 29 12:51:55 2016 -0400

    Use const &.
---
 src/RowPainter.cpp |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/RowPainter.cpp b/src/RowPainter.cpp
index 5b3e647..a50d8f8 100644
--- a/src/RowPainter.cpp
+++ b/src/RowPainter.cpp
@@ -387,7 +387,7 @@ void RowPainter::paintFirst() const
 
 void RowPainter::paintLabel() const
 {
-       docstring const str = par_.labelString();
+       docstring const & str = par_.labelString();
        if (str.empty())
                return;