[LyX/2.3.x] Fix crash when citeengine is unknown.

[Richard Heck]

commit 0dfca93139d15e851725bd6d9d6098ef09c1ca8d
Author: Richard Heck <rgh...@lyx.org>
Date:   Mon Feb 12 16:26:27 2018 -0500

    Fix crash when citeengine is unknown.
    
    (cherry picked from commit 5ee3396459602e0982234cab064c5c960af7e4fc)
---
 src/frontends/qt4/GuiDocument.cpp |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/src/frontends/qt4/GuiDocument.cpp 
b/src/frontends/qt4/GuiDocument.cpp
index 524ea39..0920378 100644
--- a/src/frontends/qt4/GuiDocument.cpp
+++ b/src/frontends/qt4/GuiDocument.cpp
@@ -4007,6 +4007,12 @@ bool GuiDocument::isBiblatex() const
                biblioModule->citeEngineCO->itemData(
                                
biblioModule->citeEngineCO->currentIndex()).toString();
 
+       // this can happen if the cite engine is unknown, which can happen
+       // if one is using a file that came from someone else, etc. in that
+       // case, we crash if we proceed.
+       if (engine.isEmpty())
+           return false;
+
        return theCiteEnginesList[fromqstr(engine)]->getCiteFramework() == 
"biblatex";
 }