meta/msaa-blit: consider weird sample count case unreachable

Suppresses a bunch of warning noise about sample_map possibly being used
uninitialized.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/drivers/common/meta_blit.c

@@ -134,6 +134,7 @@ setup_glsl_msaa_blit_scaled_shader(struct gl_context *ctx,
       break;
    default:
       _mesa_problem(ctx, "Unsupported sample count %d\n", samples);
+      unreachable("Unsupported sample count");
    }
 
    /* Create sample map string. */