ofGLReadyCallback() updated

Updated ofGLReadyCallback() to fix crash on check for GLSL version when the OpenGL context version is < 2.0.
Update to ofGLReadyCallback()

Changed ofGLReadyCallback() to check for null pointer of GLSL version string instead of checking OpenGL version and proceeding from there.
Merge pull request #2956 from hardmanko/glsl-check-crash-fix

ofGLReadyCallback() updated