array.c: report correct memsize for shared root arrays

For a shared array root, struct RArray::as.heap.aux.capa stores the
number of Arrays holding reference to that T_ARRAY instead of the actual
heap-allocated capacity. Use ARY_CAPA() macro which handles this
appropriately.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58332 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -555,7 +555,7 @@ RUBY_FUNC_EXPORTED size_t
 rb_ary_memsize(VALUE ary)
 {
     if (ARY_OWNS_HEAP_P(ary)) {
-	return RARRAY(ary)->as.heap.aux.capa * sizeof(VALUE);
+	return ARY_CAPA(ary) * sizeof(VALUE);
     }
     else {
 	return 0;