Fix to read.ncdfFlowSet: pd is now initialised before being referenced.

pd was being referenced before being initialised.
This would lead to the following error when calling read.ncdFlowSet with the phenoData argument:

```r
Error in sampleNames(pd) :
  error in evaluating the argument 'object' in selecting a method for
  function 'sampleNames': Error: object 'pd' not found
```

The fix involved simply swapping two lines around.