Fix a bug which occurs when the heading has no children

[asmsuechan]

Hi. I hit an error TypeError: Cannot read property 'type' of undefined when I write such a markdown below:

# hoge

##

[coveralls]

Coverage increased (+1.04%) to 100.0% when pulling bdc81f5 on asmsuechan:fix-a-bug into 64a3eb3 on medfreeman:master.

[asmsuechan]

Hi, @medfreeman. What do you think?

[medfreeman]

Thank you !

[thematlin]

@medfreeman I'm experiencing the same bug. Would it be possible to get this released and published to npm? Would be greatly appreciated.

[leopoldchen]

@asmsuechan @medfreeman
the solution should be like this

if (heading.children &&
    heading.children.length > 0 &&
    heading.children[0].type === "link_open") 

because heading.children might be null as well

here is an online demo

Just enable the TOC, and type a '#' on a new line, and see the error log in the console