[LyX/master] Handle negative values gracefully.

[Jean-Marc Lasgouttes]

commit c8a829d273020d10ae5a22eb8a9fabafe7b81e6e
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Tue Dec 6 16:06:03 2016 +0100

    Handle negative values gracefully.
    
    It may happen (for example with on the fly spell checking) that
    GuiFontMetrics::pos2x is called with a negative pos value. Add a
    special case for this situation.
    
    Fixes bug #10506
---
 src/frontends/qt4/GuiFontMetrics.cpp |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index 77cda80..d3c89f1 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -205,6 +205,8 @@ GuiFontMetrics::getTextLayout(docstring const & s, QFont 
font,
 int GuiFontMetrics::pos2x(docstring const & s, int const pos, bool const rtl,
                           double const wordspacing) const
 {
+       if (pos <= 0)
+               return rtl ? width(s) : 0;
        QTextLayout const & tl = getTextLayout(s, font_, rtl, wordspacing);
        /* Since QString is UTF-16 and docstring is UCS-4, the offsets may
         * not be the same when there are high-plan unicode characters