[wpe-2.38] Fix crash upon trying to add fence sync

[Scony]

This PR attempts fixing #1239 just by adding simple nullptr check. I'm not sure if this is enough i.e. if it's valid that GLContext::current() may be null at this point, although judging by same check being done in the same class in TextureMapperPlatformLayerBuffer::clone() it likely may be.

Should be reviewed by @magomez

[Scony]

Superseded by 9f13ddc7498c35c1c1629401ab853109826a4485