Fix bug #10951.

The Buffer member for the remembered definition_ was not being set.

(cherry picked from commit f130f7713f)

src/mathed/InsetMathMacro.cpp

@@ -1318,4 +1318,10 @@ void InsetMathMacro::completionPosAndDim(Cursor const & cur, int & x, int & y,
 }
 
 
+void InsetMathMacro::setBuffer(Buffer & buffer)
+{
+	d->definition_.setBuffer(buffer);
+	InsetMathNest::setBuffer(buffer);
+}
+
 } // namespace lyx

src/mathed/InsetMathMacro.h

@@ -149,6 +149,8 @@ public:
 	/// This is not used for display; however whether it is mathrel determines
 	/// how to split equations intelligently.
 	MathClass mathClass() const; //override
+	/// Override so as to set Buffer for defnition_ member, too.
+	void setBuffer(Buffer &);
 
 protected:
 	friend class MathData;

src/mathed/MathData.h

@@ -178,6 +178,8 @@ public:
 	void updateMacros(Cursor * cur, MacroContext const & mc, UpdateType, int nesting);
 	///
 	void updateBuffer(ParIterator const &, UpdateType);
+	///
+	void setBuffer(Buffer & b) { buffer_ = &b; }
 
 protected:
 	/// cached values for super/subscript placement