Fixed loop in interactive number line when tick<0

Summary:
addGraphie() was still called when isValid() was false,
which could lead to an infinite loop drawing tick marks,
such as when tickStep < 0. This now no longer freezes
the page!

Test Plan:
visit test.html
add an interactive number line
set tick step to -1
can you still edit things?
yes -> yay, it's fixed!
no -> this patch is buggy!

Auditors: alex