[renderer] Check view_model entity is valid

It usually is in quake, but my test scene does not have a view model.

libs/video/renderer/gl/gl_rmain.c

@@ -160,6 +160,9 @@ static void
 R_DrawViewModel (void)
 {
 	entity_t    ent = vr_data.view_model;
+	if (!Entity_Valid (ent)) {
+		return;
+	}
 	renderer_t *renderer = Ent_GetComponent (ent.id, scene_renderer, ent.reg);
 	if (vr_data.inhibit_viewmodel
 		|| !r_drawviewmodel

libs/video/renderer/glsl/glsl_main.c

@@ -116,6 +116,9 @@ static void
 R_DrawViewModel (void)
 {
 	entity_t    ent = vr_data.view_model;
+	if (!Entity_Valid (ent)) {
+		return;
+	}
 	renderer_t *renderer = Ent_GetComponent (ent.id, scene_renderer, ent.reg);
 	if (vr_data.inhibit_viewmodel
 		|| !r_drawviewmodel

libs/video/renderer/sw/sw_rmain.c

@@ -377,6 +377,9 @@ R_DrawViewModel (void)
 		return;
 
 	viewent = vr_data.view_model;
+	if (!Entity_Valid (viewent)) {
+		return;
+	}
 
 	renderer_t *renderer = Ent_GetComponent (viewent.id, scene_renderer,
 											 viewent.reg);

libs/video/renderer/vulkan/vulkan_main.c

@@ -108,6 +108,9 @@ static void
 Vulkan_DrawViewModel (vulkan_ctx_t *ctx)
 {
 	entity_t    ent = vr_data.view_model;
+	if (!Entity_Valid (ent)) {
+		return;
+	}
 	renderer_t *renderer = Ent_GetComponent (ent.id, scene_renderer, ent.reg);
 	if (vr_data.inhibit_viewmodel
 		|| !r_drawviewmodel