treat PC for gc events

ADD_PCs moved. That didn't change vast majority of event hooks
because vm_traece() has been placed before ADD_PC. However for GC
events the situation is different. We have to take care.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62052 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -1819,7 +1819,10 @@ rb_objspace_set_event_hook(const rb_event_flag_t event)
 static void
 gc_event_hook_body(rb_execution_context_t *ec, rb_objspace_t *objspace, const rb_event_flag_t event, VALUE data)
 {
+    /* increment PC because source line is calculated with PC-1 */
+    ec->cfp->pc++;
     EXEC_EVENT_HOOK(ec, event, ec->cfp->self, 0, 0, 0, data);
+    ec->cfp->pc--;
 }
 
 #define gc_event_hook_available_p(objspace) ((objspace)->flags.has_hook)