Throw a custom exception if Print::delete_object() gets an out-of-range identifier.

xs/src/libslic3r/Print.cpp

@@ -77,6 +77,8 @@ void
 Print::delete_object(size_t idx)
 {
     PrintObjectPtrs::iterator i = this->objects.begin() + idx;
+    if (i >= this->objects.end()) 
+        throw InvalidObjectException();
     
     // before deleting object, invalidate all of its steps in order to 
     // invalidate all of the dependent ones in Print

xs/src/libslic3r/Print.hpp

@@ -16,8 +16,12 @@
 #include "SlicingAdaptive.hpp"
 #include "LayerHeightSpline.hpp"
 
+#include <exception>
+
 namespace Slic3r {
 
+class InvalidObjectException : public std::exception {};
+
 class Print;
 class PrintObject;
 class ModelObject;