archive/ArchiveList, input/Registry: avoid zero-sized array

Some more `-Wpedantic` fixups.

src/input/Registry.cxx

@@ -83,7 +83,11 @@ const InputPlugin *const input_plugins[] = {
 	nullptr
 };
 
-bool input_plugins_enabled[std::size(input_plugins) - 1];
+static constexpr std::size_t n_input_plugins = std::size(input_plugins) - 1;
+
+/* the std::max() is just here to avoid a zero-sized array, which is
+   forbidden in C++ */
+bool input_plugins_enabled[std::max(n_input_plugins, std::size_t(1))];
 
 bool
 HasRemoteTagScanner(const char *uri) noexcept