audioread race condition freezes when cleaning up

I have a flac file that causes acoustid to freeze when doing:
```
import acoustid
acoustid.fingerprint_file('01.flac')
```
If I press Ctrl-C, I get the following backtrace:
```
Traceback (most recent call last):
File "/usr/lib64/python3.6/threading.py", line 916, in _bootstrap_inner
self.run()
File "/usr/lib/python3.6/site-packages/audioread/gstdec.py", line 149, in run
self.loop.run()
File "/usr/lib64/python3.6/site-packages/gi/overrides/GLib.py", line 588, in run
raise KeyboardInterrupt
```
Btw, the file plays perfectly fine with ffplay, mpv, mplayer... Also, I tried reading the audio blocks with simple audioread code like:
```
with audioread.audio_open(path) as f:
for x in f:
print(x)
```
and that seems to work fine.

What makes me think that it's an audioread issue and not an acoustid issue is that in audioread's gstdec.py, if I put `return None` as the first line of get_loop_thread, it doesn't freeze (works normally).

I've tried to debug a bit the problem and noticed that acoustid has a _fingerprint_file_audioread function defined as:
```
def _fingerprint_file_audioread(path, maxlength):
"""Fingerprint a file by using audioread and chromaprint."""
try:
with audioread.audio_open(path) as f:
duration = f.duration
fp = fingerprint(f.samplerate, f.channels, iter(f), maxlength)
except audioread.DecodeError:
raise FingerprintGenerationError("audio could not be decoded")
return duration, fp
```
If I add `time.sleep(0.00000000000001)` at the end of the `with` block (just after the fingerprint call), sometimes it works as expected, and if I use `time.sleep(0.1)` instead, it seems to "fix it" and doesn't freeze anymore, so it seems to be some kind of race condition.

After some more debugging I found out that the exact line where it's freezing is in the call to `self.pipeline.set_state(Gst.State.NULL)` inside `GstAudioFile.close` but I'm not sure how to continue from there since I don't have much experience with gstreamer.

I'm using audioread 2.1.5, gstreamer 1.12.3 and pyacoustid 1.1.5 with chromaprint 1.4.2 . I also tried with gstreamer 1.12.2 and audioread 2.1.4.

Btw, another solution I found was to do `ffmpeg -i 01.flac output.flac`. The resulting file is processed correctly and never freezes.

Start by reproducing the FLAC case through acoustid.fingerprint_file and the audioread loop in audioread/gstdec.py. Inspect get_loop_thread and GstAudioFile.close, especially the pipeline.set_state(Gst.State.NULL) call, using the reported backtrace and timing behavior. Done means cleanup completes without freezing while processing the problematic file.

Written by the indexing model from the issue text.