Catch malformed row error
Catch malformed row error

Hey,
When a table row have an undefined cell, 
``` if (!data._span) { ``` produce an exception.

When you have a lot of array in a pdf, it s totaly insame to debug it.
I suggest to catch it and throw some information to help the dev to debug that.
bavellone/pdfmake,TW-QEN/pdfmake,bavellone/pdfmake,dennisrjohn/pdfmake,TW-QEN/pdfmake,dennisrjohn/pdfmake