Fixed erroneous FRAMEBUFFER_UNSUPPORTED.

WebGL spec states: "If framebuffer is null, the default framebuffer
provided by the context is bound." Thus there is no need the check if
we have a self-bound framebuffer before querying its status from the
system.

Change-Id: I389d3e3b81ac61c6498a76ae07303caf75e2787e
Task-number: QTBUG-45197
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>