Protect against GC when constructing a System

With Guile 2.2, it's possible to observe crashes like
    #0  std::vector<Grob*, std::allocator<Grob*> >::size() const (this=0x0)
    #1  System::derived_mark() const (this=0x556efd94f460) at [...]/lily/system.cc:113
    #2  0x0000556ef6b08f25 in Grob::mark_smob() const (this=0x556efd94f460)
    #3 to #10 in libguile-2.2.so.1 and libgc.so.1
    #11 0x00007f0e322bc875 in scm_i_new_smob ()
    #12 0x0000556ef6b0100b in scm_new_smob (data=93935189161312, tc=13943)
    #13 Smob_base<Grob_array>::register_ptr(Grob_array*) (p=0x556efd94f560)
    #14 Simple_smob<Grob_array>::smobbed_copy() const (this=<synthetic pointer>)
    #15 Grob_array::make_array()
    #16 0x0000556ef6c72bdf in System::init_elements() (this=0x556efd94f460)
    #17 0x0000556ef6bfb3f9 in Score_engraver::initialize() (this=0x556efc946cd0)

In this case, System::init_elements() calls Grob_array::make_array()
to later initialize all_elements_, which is consequently 0 until then.

(cherry picked from commit aacb2da)

lily/system.cc

@@ -108,9 +108,12 @@ System::typeset_grob (Grob *elem)
 void
 System::derived_mark () const
 {
-  const vector <Grob *> &arr = all_elements_->array ();
-  for (vsize i = arr.size (); i--;)
-    scm_gc_mark (arr[i]->self_scm ());
+  if (all_elements_)
+    {
+      const vector <Grob *> &arr = all_elements_->array ();
+      for (vsize i = arr.size (); i--;)
+        scm_gc_mark (arr[i]->self_scm ());
+    }
 
   if (pscore_)
     scm_gc_mark (pscore_->self_scm ());