mesa/main: In _mesa_CompressedTextureSubImage3D() check found texObj

Check returned texObj is not null. If texObj is null there is already
GL_INVALID_OPERATION error set.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>

src/mesa/main/teximage.c

@@ -4732,6 +4732,8 @@ _mesa_CompressedTextureSubImage3D(GLuint texture, GLint level, GLint xoffset,
 
    texObj = _mesa_lookup_texture_err(ctx, texture,
                                      "glCompressedTextureSubImage3D");
+   if (!texObj)
+      return;
 
    _mesa_compressed_texture_sub_image(ctx, 3, texObj, texObj->Target, level,
                                       xoffset, yoffset, zoffset,