Merge pull request #284 from morrolinux/CTRL-Z-deletion-fix

[BUGFIX] - CTRL-Z deletion recovery segfault

src/model/Layer.cpp

@@ -82,6 +82,9 @@ void Layer::insertElement(Element* e, int pos)
 		}
 	}
 
+	if(pos == -1) pos=0; 	//otherwise it segfaults on the next step trying
+				//to access a negative index on the elements array 
+
 	this->elements.insert(this->elements.begin() + pos, e);
 }