bi: Fix --report SIGSEGV caused by uninitialised video

At some point in 1.17.x, the check for whether the game window
was null disappeared. This brings it back by exposing a check
through the video namespace so as to avoid piggybacking further
on the video::get_window() function, which we are meant to avoid
using.

src/build_info.cpp

@@ -550,6 +550,10 @@ list_formatter video_settings_report_internal(const std::string& heading = "")
 		placeholder = "Running in non-interactive mode.";
 	}
 
+	if(!video::has_window()) {
+		placeholder = "Video not initialized yet.";
+	}
+
 	if(!placeholder.empty()) {
 		fmt.set_placeholder(placeholder);
 		return fmt;

src/video.cpp

@@ -397,6 +397,11 @@ void init_window()
 	update_framebuffer();
 }
 
+bool has_window()
+{
+	return bool(window);
+}
+
 point output_size()
 {
 	if (testing_) {

src/video.hpp

@@ -85,6 +85,9 @@ bool testing();
 /* Windowing functions */
 /***********************/
 
+/** Whether the game has set up a window to render into */
+bool has_window();
+
 /** Whether we are currently in fullscreen mode */
 bool is_fullscreen();