Fix segfault if GL_OES_mapbuffer extension is missing.

QOpenGLBuffer::map() returns an invalid pointer if GL_OES_mapbuffer
extension is missing. Adding check for extension before calling this
function.

Change-Id: I126733b05a7ee66343362f12283219aabb506bc5
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>