[LyX/master] Fix for crash/segfault introduced with [244de5d2/lyxgit], thanks Guillame.

[Tommaso Cucinotta]

commit 31adae1f23c4a2767b4307d2c3e59972826b6805
Author: Tommaso Cucinotta <tomm...@lyx.org>
Date:   Thu Apr 20 23:39:31 2017 +0200

    Fix for crash/segfault introduced with [244de5d2/lyxgit], thanks Guillame.
---
 src/graphics/GraphicsCacheItem.cpp |    2 +-
 src/graphics/GraphicsConverter.cpp |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/graphics/GraphicsCacheItem.cpp 
b/src/graphics/GraphicsCacheItem.cpp
index 4ed496a..306dcdb 100644
--- a/src/graphics/GraphicsCacheItem.cpp
+++ b/src/graphics/GraphicsCacheItem.cpp
@@ -97,7 +97,7 @@ public:
        /// The filename we refer too.
        FileName const filename_;
        /// The document filename this graphic item belongs to
-       FileName const & doc_file_;
+       FileName const doc_file_;
        ///
        ActiveFileMonitorPtr monitor_;
 
diff --git a/src/graphics/GraphicsConverter.cpp 
b/src/graphics/GraphicsConverter.cpp
index 84cbaf9..63b6a44 100644
--- a/src/graphics/GraphicsConverter.cpp
+++ b/src/graphics/GraphicsConverter.cpp
@@ -63,7 +63,7 @@ public:
        SignalType finishedConversion;
 
        ///
-       FileName const & doc_fname_;
+       FileName const doc_fname_;
        ///
        string script_command_;
        ///