Glass in general can take much higher heat than the low-temp ovens we use to cure polymer clay, but some
crack under the right circumstances. Just putting clay over any glass (or over other surface) also buffers the temp from spiking**, and insulates it if not baking a long time.
So if possible:
...don't change temps quickly (especially letting it cool gradually in the oven and especially not putting down on a cold or wet surface while hot)...you could even use a "completely enclosed" baking method
...put the item in when the oven is preheated unless you're positive that in your particular oven the normal spiking of temperature the thermostat usually does to reach the temp you've set on the dial won't be so high that it becomes a problem
...cover the glass completely, or mostly, with clay (to insulate, buffer)
...strangely enough, be more careful with thick glass than thinner glass
But for what you're doing, you shouldn't have any problems at all and I wouldn't worry about it.
**pre-coating the glass with a permanent white glue (then allowing to dry, or tack up well) will also act as a buffering layer since it's flexible/elastic --that's normally done to give a bit less-slick and a tackier surface for the clay to grab onto while raw
You might also want to check out the Covering
pages of my site for more details on those topics:http://glassattic.com/polymer/covering.htm
(click on Glass & Ceramic
P.S. If you're completely covering glass (front, back and sides) in some area, the clay itself could crack though because polymer clay shrinks ever so slightly when cured but glass will stay the same. So if you're using a thin layer of clay, covering a large-ish area of glass, and not using the glue layer, you could have that problem (but not what you're doing).