tweaks: switch to the preceding buffer in a cheaper way (when in help)

When a help-text buffer is closed while exiting from the help viewer,
the rest of the exit code takes care that the preceding buffer gets
displayed properly.  And when a help-text buffer is closed because a
SIGWINCH occurred, doing anything for the preceding buffer is a waste
of time because a new help-text buffer will be opened and displayed
instantly.

src/help.c

@@ -239,7 +239,7 @@ void do_help(void)
 #endif
 
 	/* Switch back to the buffer we were invoked from. */
-	switch_to_prev_buffer();
+	openfile = openfile->prev;
 
 	if (ISSET(NO_HELP)) {
 		currmenu = oldmenu;

src/winio.c

@@ -3375,7 +3375,7 @@ void total_refresh(void)
 #ifdef ENABLE_HELP
 	if (inhelp) {
 		close_buffer();
-		switch_to_prev_buffer();
+		openfile = openfile->prev;
 		wrap_help_text_into_buffer();
 	} else
 #endif