GlES2: Add NULL check

Check for NULL before calling thread specific glGetString.

CRs-fixed: 569609

Change-Id: Ifee8257152ba7ca7a05df5d7b113171b71d16c82
surfaceflinger: Set max acquired buffer count

If triple framebuffers are enabled, the number of acquired
buffers isn't increased by default. Set the count to one less
than the max buffers to make sure all three buffers come into
play.

Change-Id: I65054ed4004607f5be68c893f6f621e821cfbf1c