fixes bug 1017077 - Only make links to home page of valid product names

@rhelmer r?
The important change is that the nav (which is part if crashstats_base.html) build up their links like this `{% url('someview', product) %}` that `product` is a "context variable" that almost every view has. However, what it used to do here in `report_index()` was to override that with the product that was found in the dump.