Memory leak with empty arrays

[ankane]

Hi, it looks like there's a memory leak for arrays that have no elements.

Repro command with ruby_memcheck

ruby_memcheck -e 'require "numo/narray"; Numo::SFloat.new(0, 3)'

Output

16 bytes in 1 blocks are definitely lost in loss record 2 of 33
  malloc (at /usr/libexec/valgrind/vgpreload_memcheck-arm64-linux.so)
  rb_gc_impl_malloc (default.c:8294)
  ruby_xmalloc2_body (gc.c:5221)
  ruby_xmalloc2 (gc.c:5215)
 *na_alloc_shape (narray.c:303)
 *na_setup_shape (narray.c:313)
 *na_setup (narray.c:335)
 *na_initialize (narray.c:394)
  vm_call_cfunc_with_frame_ (vm_insnhelper.c:3902)
  vm_sendish.constprop.0 (vm_insnhelper.c:6124)
  vm_exec_core (insns.def:904)
  vm_exec_loop (vm.c:2825)
  rb_vm_exec (vm.c:2804)
  rb_ec_exec_node (eval.c:283)
  ruby_run_node (eval.c:321)
  rb_main (main.c:42)
  main (main.c:62)