Fix 0plus0

Consider the following:
```
>> f = chebfun(0)+0;
>> f.funs{1}.onefun
ans = 
  chebtech2 with properties:

      coeffs: 0
      vscale: 0
      hscale: 5.000000000000000e-01
     ishappy: 1
    epslevel: Inf
```
The epslevel is just mind-boggling.

With a solution provided by @aaustin141 we have "fixed" this. The problem was basically a divide by zero bug happening in chebtech/plus. In general, epslevels continue to be a tricky business and here again, since both functions were 0, the vscale of both is zero and a manual update to epslevel in chebtech/plus produced an inf. This benign looking bug was causing all sorts of problems for the work I am doing on coefficient re-ordering of Chebfun.