fix(libssh2-runner): fix slow stderr read when no stdout

In case there's only stderr stream and no data on stdout, stderr stream
processing waits for stdout processing timeout (0.5s by default). This
causes long processing of many stderr lines as this delay happens for
each line.

Fix by verifying first if given stream has data and proceed to next if
not.

refs: scylladb/java-driver#258

sdcm/remote/libssh2_client/__init__.py

@@ -440,25 +440,30 @@ def _process_output(  # pylint: disable=too-many-arguments, too-many-branches
             if perf_counter() > end_time:
                 reader.stop()
                 return False
-            if stdout_stream is not None:
+
+            data_processed = False
+            if stdout_stream is not None and reader.stdout.qsize():
                 try:
                     stdout = reader.stdout.get(timeout=timeout_read_data_chunk)
                     data = stdout.decode(encoding, errors='ignore') + '\n'
                     stdout_stream.write(data)
                     for watcher in watchers:
                         watcher.submit_line(data)
+                    data_processed = True
+                except Exception:  # pylint: disable=broad-except  # noqa: BLE001
+                    pass
+            if stderr_stream is not None and reader.stderr.qsize():
+                try:
+                    stderr = reader.stderr.get(timeout=timeout_read_data_chunk)
+                    data = stderr.decode(encoding) + '\n'
+                    stderr_stream.write(data)
+                    for watcher in watchers:
+                        watcher.submit_line(data)
+                    data_processed = True
                 except Exception:  # pylint: disable=broad-except  # noqa: BLE001
                     pass
-            if stderr_stream is not None:
-                if reader.stderr.qsize():
-                    try:
-                        stderr = reader.stderr.get(timeout=timeout_read_data_chunk)
-                        data = stderr.decode(encoding) + '\n'
-                        stderr_stream.write(data)
-                        for watcher in watchers:
-                            watcher.submit_line(data)
-                    except Exception:  # pylint: disable=broad-except  # noqa: BLE001
-                        pass
+            if not data_processed:  # prevent unbounded loop in case no data on the streams
+                sleep(timeout_read_data_chunk or 0.5)
         return True
 
     @staticmethod