[model] Reset mod_numknown when clearing models

Vulkan actually needs Mod_ClearAll, and mod_numknown not getting reset
was the actual cause of the segfault.

libs/models/model.c

@@ -178,6 +178,7 @@ Mod_ClearAll (void)
 	for (i = 0; i < mod_numknown; i++) {
 		mod_unload_model (i);
 	}
+	mod_numknown = 0;
 }
 
 model_t *

libs/video/renderer/vid_render_vulkan.c

@@ -725,6 +725,7 @@ vulkan_vid_render_shutdown (void)
 	QFV_DeviceWaitIdle (device);
 
 	SCR_Shutdown ();
+	Mod_ClearAll ();
 
 	Vulkan_Compose_Shutdown (vulkan_ctx);
 	Vulkan_Translucent_Shutdown (vulkan_ctx);