ci: Avoid deadlock while reading uncrustify output

The docu for `Popen.wait()` says:
> This will deadlock when using stdout=PIPE and/or stderr=PIPE and
> the child process generates enough output to a pipe such that it
> blocks waiting for the OS pipe buffer to accept more data.

Fixes 3caa5fea3c

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1971>

check-style.py

@@ -76,11 +76,11 @@ def reformat_chunks(chunks, rewrite):
 
         # uncrustify chunk
         proc = subprocess.Popen(["uncrustify", "-c", uncrustify_cfg, "-f", tmp.name], stdout=subprocess.PIPE)
+        reindented = proc.stdout.readlines()
         proc.wait()
         if proc.returncode != 0:
             continue
 
-        reindented = proc.stdout.readlines()
         tmp.close()
 
         # Remove INDENT-ON/OFF comments