stream_reader() hangs (keeps spinning on CPU) when empty buffer is supplied

Hi there!

We've run into an issue where bad input data caused the library to keep spinning in `decompression_reader.c:370` as seen on the screenshot from `py-spy` stacktrace dump.

Here's the code to reproduce this situation:

```
import zstandard
import io

decompressor = zstandard.ZstdDecompressor()
bad_input = b''

with decompressor.stream_reader(bad_input) as decompressing_reader:
with io.TextIOWrapper(decompressing_reader, encoding='utf-8', newline='\n') as reader:
for line in reader:
print(line)
```

Start at decompression_reader.c:370 and reproduce the hang with the Python example using an empty buffer and stream_reader(). Trace how the empty input is handled, then verify that the example terminates without spinning on CPU.

Written by the indexing model from the issue text.