gldisplay: do not call glCheckFramebufferStatus if 0 is bound
gldisplay: do not call glCheckFramebufferStatus if 0 is bound

Indeed it may return GL_FRAMEBUFFER_UNDEFINED if there is no
default framebuffer.
It seems to be the case on MacOSX

mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl