Provide a tor_process_get_stderr_pipe() function

We already have the rest of the utilities needed for handling stderr
handles. Just need the actual higher-level function to get the pipe.

This is needed for #9957.

src/common/util.c

@@ -3600,13 +3600,26 @@ tor_process_get_stdout_pipe(process_handle_t *process_handle)
 {
   return process_handle->stdout_pipe;
 }
+
+HANDLE
+tor_process_get_stderr_pipe(process_handle_t *process_handle)
+{
+  return process_handle->stderr_pipe;
+}
 #else
 /* DOCDOC tor_process_get_stdout_pipe */
 FILE *
 tor_process_get_stdout_pipe(process_handle_t *process_handle)
 {
   return process_handle->stdout_handle;
 }
+
+/* DOCDOC tor_process_get_stderr_pipe */
+FILE *
+tor_process_get_stderr_pipe(process_handle_t *process_handle)
+{
+  return process_handle->stderr_handle;
+}
 #endif
 
 /* DOCDOC process_handle_new */

src/common/util.h

@@ -488,8 +488,10 @@ char *tor_join_win_cmdline(const char *argv[]);
 int tor_process_get_pid(process_handle_t *process_handle);
 #ifdef _WIN32
 HANDLE tor_process_get_stdout_pipe(process_handle_t *process_handle);
+HANDLE tor_process_get_stderr_pipe(process_handle_t *process_handle);
 #else
 FILE *tor_process_get_stdout_pipe(process_handle_t *process_handle);
+FILE *tor_process_get_stderr_pipe(process_handle_t *process_handle);
 #endif
 
 #ifdef _WIN32