exrstdattr: Headers are not initialized from the input file

It looks like that as part of https://github.com/openexr/openexr/commit/ecc9fc53c954e2430539004f8ca1c8ce35ce43b1, the vector of headers to which new attributes are added switched to:

```
   MultiPartInputFile in (inFileName);
   int numParts = in.parts();
   vector <Header> headers (numParts);
```
And doesn't seem to ever be initialized with the header from the input file parts. This seems to cause issues with calls to ```Header::type()```. In it's current state, ```exrstdattr``` doesn't seem to even work, at least for me.