TagListPage: fix nullptr dereference when adding "[All]"

Commit 9f5a075 caused a regression because two variables were mixed
up: the `text` must not be nullptr, or else Utf8ToLocale() crashes.

Closes #77

NEWS

@@ -1,5 +1,6 @@
 ncmpc 0.41 - not yet released
 * new main loop (copying code from MPD, replacing boost::asio)
+* library page: fix crash bug
 * lyrics: remove the "lyricwiki" plugin because the site is gone
 
 ncmpc 0.40 - (2020-10-07)

src/TagListPage.cxx

@@ -178,8 +178,10 @@ add_query(struct mpdclient *c, const TagFilter &filter,
 		return;
 
 	const char *text = value;
-	if (value == nullptr)
-		value = filter.empty() ? "?" : filter.front().second.c_str();
+	if (text == nullptr)
+		/* adding the special "[All]" entry: show the name of
+		   the previous level in the filter */
+		text = filter.empty() ? "?" : filter.front().second.c_str();
 
 	screen_status_printf(_("Adding \'%s\' to queue"),
 			     Utf8ToLocale(text).c_str());