OpenGL: remove potential NULL dereference
OpenGL: remove potential NULL dereference

(cherry picked from commit 267d8be5f35b8a33bf5e634ff5087a8ab9788019)
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2