Bug#861736: Still some regression in python-nxs

[Carlos Pascual]

Hi,

while nexus 4.3.2-svn1921-5 fixes the originally reported exception, the
fix seems not to be complete [1].

In order to trigger the still-existing regression, run the following
python code (which ideally should exit without errors):

```
import nxs
f = nxs.open("/tmp/foo.h5", "w5")
f.makegroup('entry', 'NXentry')
f.opengroup('entry')
f.makegroup('g', 'NXcollection')
f.opengroup('g', 'NXcollection')
f.makedata('d', 'float64', shape=(1,))
f.opendata('d')
f.putdata(1.23)
f.closedata()
f.closegroup()
f.flush()
f.close()
```

In stretch + nexus_4.3.2-svn1921-5, it fails with the following
exception:

```
Traceback (most recent call last):
  File "nxs_flush_bug.py", line 12, in <module>
    f.flush()
  File "/usr/lib/python2.7/dist-packages/nxs/napi.py", line 397, in flush
    raise NeXusError, "Could not flush NeXus file %s"%(self.filename)
nxs.napi.NeXusError: Could not flush NeXus file /tmp/foo.h5
```

The same code works well when run on a clean jessie (I used an official
debian:stable docker) and it fails with the same exception as above when
run on sid (I used the official debian:unstable docker)

Cheers,

Carlos

[1] I am replying here because this new problem seems likely to be related
with 861736, but if you think that it may be an unrelated bug I'd be glad
to open a new bug