Assert that an output stream is available when we expect it (#541)

A new version of pytype will begin erroring on this. Add an assertion so
pytype can assume that p.stdout is not None in this case.

Author: boomanaiden154

compiler_opt/rl/compilation_runner.py

@@ -258,6 +258,7 @@ def start_cancellable_process(cmdline: list[str],
       raise subprocess.CalledProcessError(retcode, cmdline)
     else:
       if want_output:
+        assert p.stdout is not None
         ret: bytes = p.stdout.read()
         p.stdout.close()
         return ret