decode: validate shader type

Fix found thanks to american fuzzy lop.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

src/vrend_decode.c

@@ -219,6 +219,10 @@ static int vrend_decode_set_constant_buffer(struct vrend_decode_ctx *ctx, uint16
 
    shader = get_buf_entry(ctx, VIRGL_SET_CONSTANT_BUFFER_SHADER_TYPE);
    index = get_buf_entry(ctx, VIRGL_SET_CONSTANT_BUFFER_INDEX);
+
+   if (shader >= PIPE_SHADER_TYPES)
+      return EINVAL;
+
    vrend_set_constants(ctx->grctx, shader, index, nc, get_buf_ptr(ctx, VIRGL_SET_CONSTANT_BUFFER_DATA_START));
    return 0;
 }