hand over udata to h5->mem a little earlier
hand over udata to h5->mem a little earlier

I think udata will be freed by the calback as soon as the callbacks are
set and H5Pclose is called, so the backward link should be set up at
that place.

Without this change, local_image_free may be called without a proper
reference to udata, which may raise an assertion:
assert(udata->fapl_image_ptr == udata->app_image_ptr);
while trying to open a broken file.

Unidata/netcdf-c,Unidata/netcdf-c,Unidata/netcdf-c,Unidata/netcdf-c