[LyX/master] Make InsetMathNest::editXY more robust

[Jean-Marc Lasgouttes]

commit a69bae0c99e1d365b1fe59693274ba7f608e1829
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Fri Feb 24 23:58:53 2017 +0100

    Make InsetMathNest::editXY more robust
    
    If no reasonable cell has been found to put the cursor in (maybe
    because the cell exist but is not in the cache), exit gracefully.
    
    Fixes bug #10520
---
 src/mathed/InsetMathNest.cpp |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/src/mathed/InsetMathNest.cpp b/src/mathed/InsetMathNest.cpp
index 1f0adda..596e32a 100644
--- a/src/mathed/InsetMathNest.cpp
+++ b/src/mathed/InsetMathNest.cpp
@@ -1495,7 +1495,7 @@ void InsetMathNest::edit(Cursor & cur, bool front, 
EntryDirection entry_from)
 
 Inset * InsetMathNest::editXY(Cursor & cur, int x, int y)
 {
-       int idx_min = 0;
+       int idx_min = -1;
        int dist_min = 1000000;
        for (idx_type i = 0, n = nargs(); i != n; ++i) {
                int const d = cell(i).dist(cur.bv(), x, y);
@@ -1504,6 +1504,9 @@ Inset * InsetMathNest::editXY(Cursor & cur, int x, int y)
                        idx_min = i;
                }
        }
+       if (idx_min == -1)
+               return this;
+
        MathData & ar = cell(idx_min);
        cur.push(*this);
        cur.idx() = idx_min;