cont.c (fiber_memsize): do not rely on ROOT_FIBER_CONTEXT

We can check if the fiber we're interested in is the
th->root_fiber for the owner thread, so there is no need to use
ROOT_FIBER_CONTEXT.

Note: there is no guarantee th->ec points to
&th->root_fiber->cont.saved_ec, thus vm::thread_memsize may not
account for root fiber correctly (pre-existing bug).

[Bug #15050]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64705 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -493,12 +493,15 @@ static size_t
 fiber_memsize(const void *ptr)
 {
     const rb_fiber_t *fib = ptr;
-    size_t size = 0;
+    size_t size = sizeof(*fib);
+    const rb_execution_context_t *saved_ec = &fib->cont.saved_ec;
+    const rb_thread_t *th = rb_ec_thread_ptr(saved_ec);
 
-    size = sizeof(*fib);
-    if (fib->cont.type != ROOT_FIBER_CONTEXT &&
-	fib->cont.saved_ec.local_storage != NULL) {
-	size += st_memsize(fib->cont.saved_ec.local_storage);
+    /*
+     * vm.c::thread_memsize already counts th->ec->local_storage
+     */
+    if (saved_ec->local_storage && fib != th->root_fiber) {
+	size += st_memsize(saved_ec->local_storage);
     }
     size += cont_memsize(&fib->cont);
     return size;