:name: option for math causes a crash

[tk0miya]

Describe the bug
:name: option for math directive causes a crash on v1.8.
I can built it successfully with v1.7.

To Reproduce
Put following mark up to your document:

.. math:: x = 1
   :name: test

and build it.

Traceback (most recent call last):
  File "/Users/tkomiya/work/sphinx/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/Users/tkomiya/work/sphinx/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 347, in build_update
    len(to_build))
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 412, in build
    self.write(docnames, list(updated_docnames), method)
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 593, in write
    self._write_serial(sorted(docnames))
  File "/Users/tkomiya/work/sphinx/sphinx/builders/__init__.py", line 604, in _write_serial
    self.write_doc(docname, doctree)
  File "/Users/tkomiya/work/sphinx/sphinx/builders/html.py", line 731, in write_doc
    self.docwriter.write(doctree, destination)
  File "/Users/tkomiya/work/tmp/lib/python3.6/site-packages/docutils/writers/__init__.py", line 80, in write
    self.translate()
  File "/Users/tkomiya/work/sphinx/sphinx/writers/html.py", line 58, in translate
    self.document.walkabout(visitor)
  File "/Users/tkomiya/work/tmp/lib/python3.6/site-packages/docutils/nodes.py", line 174, in walkabout
    if child.walkabout(visitor):
  File "/Users/tkomiya/work/tmp/lib/python3.6/site-packages/docutils/nodes.py", line 174, in walkabout
    if child.walkabout(visitor):
  File "/Users/tkomiya/work/tmp/lib/python3.6/site-packages/docutils/nodes.py", line 166, in walkabout
    visitor.dispatch_visit(self)
  File "/Users/tkomiya/work/tmp/lib/python3.6/site-packages/docutils/nodes.py", line 1894, in dispatch_visit
    return method(node)
  File "/Users/tkomiya/work/sphinx/sphinx/writers/html.py", line 886, in visit_math_block
    visit(self, node)
  File "/Users/tkomiya/work/sphinx/sphinx/ext/mathjax.py", line 49, in html_visit_displaymath
    number = get_node_equation_number(self, node)
  File "/Users/tkomiya/work/sphinx/sphinx/util/math.py", line 28, in get_node_equation_number
    id = node['ids'][0]
IndexError: list index out of range

Expected behavior
build succeeded.

Your project
N/A

Screenshots
N/A

Environment info

• OS: Mac
• Python version: 3.6.5
• Sphinx version: 1.8.4
• Sphinx extensions: Nothing