[LyX/master] Add missing check

[Georg Baum]

commit 5bb7857bb2d50c4e1cc0ca10e66ff32236acf900
Author: Georg Baum <b...@lyx.org>
Date:   Sun Mar 15 13:56:48 2015 +0100

    Add missing check
    
    The other insert method has this as well, and it is indeed a problem is pos
    points outside the vector.

diff --git a/src/mathed/MathData.cpp b/src/mathed/MathData.cpp
index edd8089..fe73aa8 100644
--- a/src/mathed/MathData.cpp
+++ b/src/mathed/MathData.cpp
@@ -68,6 +68,7 @@ MathAtom const & MathData::operator[](pos_type pos) const
 
 void MathData::insert(size_type pos, MathAtom const & t)
 {
+       LBUFERR(pos <= size());
        base_type::insert(begin() + pos, t);
 }