Fix bug #8105: Crash when deleting math macro from the inside

It seems that parameterless macros are not wrapped into a MathAtom.
Rather than touching the macro code (which is tantamount to opening
a can of worms, IMHO), I prefer this solution.

(cherry picked from commit d07f9eec20)

src/CursorSlice.cpp

@@ -23,7 +23,7 @@
 #include "insets/Inset.h"
 
 #include "mathed/InsetMath.h"
-#include "mathed/MathData.h"
+#include "mathed/MathMacro.h"
 
 #include "support/lassert.h"
 
@@ -61,7 +61,9 @@ Paragraph & CursorSlice::paragraph() const
 pos_type CursorSlice::lastpos() const
 {
 	LASSERT(inset_, /**/);
-	return inset_->asInsetMath() ? cell().size() 
+	InsetMath const * math = inset_->asInsetMath();
+	bool paramless_macro = math && math->asMacro() && !math->asMacro()->nargs();
+	return math ? (paramless_macro ? 0 : cell().size()) 
 		    : (text()->empty() ? 0 : paragraph().size());
 }
 

status.20x

@@ -89,6 +89,9 @@ What's new
   flag by using the same latex backend used for previewing the document
   instead of always using the plain latex backend.
 
+- Fix crash when dissolving a math macro whose first argument is another
+  parameterless macro (bug 8105).
+
 
 * USER INTERFACE