Fix crash if using a custom pandoc installation

[kakaroto]

Fixes #563

[gedakc]

Following post HEAVILY edited.

When testing on Kubuntu 16.04 with this PR patch applied to current develop branch, I still experience a crash with a Pandoc custom path. <-- Encountered issue #535 instead.

See issue 563 - steps to reproduce crash.

Following is the console log:

<snip>

Perhaps I made a mistake in testing.

Can you retest?

[gedakc]

Ah, just found that the crash I'm experiencing is due to Issue 535 - pandoc export crashes if project title is empty.

I will update the test steps and re-test.

[gedakc]

After re-testing this PR with the updated steps, Manuskript no longer crashes when using a Pandoc custom path.

See issue 563 - steps to reproduce crash.

Using Ubuntu 16.04 I have confirmed that this PR fixes the crash.

If no concerns are raised then I will plan to merge this PR with the develop branch in the next day or so.

[gedakc]

'Merging this PR with the develop branch for inclusion in the next release of Manuskript.