[LyX/master] Check whether the buffer pointer is valid before using it

[Enrico Forestieri]

commit d275d034c056ea38db70c9f713d5df0b91103d62
Author: Enrico Forestieri <for...@lyx.org>
Date:   Thu Jun 25 01:25:42 2015 +0200

    Check whether the buffer pointer is valid before using it

diff --git a/src/mathed/MathMacro.cpp b/src/mathed/MathMacro.cpp
index 636d059..78ea2bb 100644
--- a/src/mathed/MathMacro.cpp
+++ b/src/mathed/MathMacro.cpp
@@ -207,7 +207,7 @@ MathMacro::MathMacro(MathMacro const & that)
                // We need to update d->macro_ by ourselves because in this case
                // MathData::metrics() is not called when selecting a math inset
                DocIterator const & pos = d->macroBackup_.pos();
-               d->macro_ = pos.buffer()->getMacro(name(), pos);
+               d->macro_ = pos.buffer() ? pos.buffer()->getMacro(name(), pos) 
: 0;
                if (!d->macro_)
                        d->macro_ = &d->macroBackup_;
        }
@@ -225,7 +225,7 @@ MathMacro & MathMacro::operator=(MathMacro const & that)
                // We need to update d->macro_ by ourselves because in this case
                // MathData::metrics() is not called when selecting a math inset
                DocIterator const & pos = d->macroBackup_.pos();
-               d->macro_ = pos.buffer()->getMacro(name(), pos);
+               d->macro_ = pos.buffer() ? pos.buffer()->getMacro(name(), pos) 
: 0;
                if (!d->macro_)
                        d->macro_ = &d->macroBackup_;
        }