Unassigned variable boxes fail crash assembly

[mikemitchel]

Had an issue where 7 pdf templates were failing to render with can't find width of undefined error. Turns out the smallest of unassigned variable boxes was on the 4th template, and was causing the above error. Error was happening here:

a2jdeps/pdf/assemble.js

Line 27 in efdce80

var scaleX = pdfSize.width / domSize.width

the fix is probably to catch the empty box at the point of assembly in the DAT where the above function is being used - aka, if a box doesn't have a variable assigned, don't call the above failing function.