sf: work around for no EGL_RENDERABLE_TYPE support

Conflicts:
	services/surfaceflinger/SurfaceFlinger.cpp

Change-Id: I04de379c73fc349526527ded9ddd3dd07f1998a0
GlES2: Add NULL check

Check for NULL before calling thread specific glGetString.

CRs-fixed: 569609

Change-Id: Ifee8257152ba7ca7a05df5d7b113171b71d16c82