Do not abort when unlinking non-data ruby objects

Fixes issue #1168.
Remove a call to abort() (introduced by commit
0e725b5d9b) made when SWIG_RubyUnlinkObjects()
is called on non T_DATA objects. It can happen when the destruction of T_DATA
objects is deferred: the Ruby GC first turn them to T_ZOMBIE, then calls their
free method (SWIG_RubyUnlinkObjects()).

Examples/ruby/free_function/runme.rb

@@ -39,7 +39,11 @@ GC.start
 # C++ object
 ok = false
 begin
-  puts tiger2.get_name
+  # Let's stress the GC a bit, a single pass might not be enough.
+  10.times {
+    GC.start
+    puts tiger2.get_name
+  }
 rescue ObjectPreviouslyDeleted => error
   ok = true
 end

Lib/ruby/rubytracking.swg

@@ -101,13 +101,14 @@ SWIGRUNTIME void SWIG_RubyRemoveTracking(void* ptr) {
 
 /* This is a helper method that unlinks a Ruby object from its
    underlying C++ object.  This is needed if the lifetime of the
-   Ruby object is longer than the C++ object */
+   Ruby object is longer than the C++ object. */
 SWIGRUNTIME void SWIG_RubyUnlinkObjects(void* ptr) {
   VALUE object = SWIG_RubyInstanceFor(ptr);
 
   if (object != Qnil) {
-    if (TYPE(object) != T_DATA)
-      abort();
+    // object might have the T_ZOMBIE type, but that's just
+    // because the GC has flagged it as such for a deferred
+    // destruction. Until then, it's still a T_DATA object.
     DATA_PTR(object) = 0;
   }
 }