asfdemux: Re-initialize metadata in PAUSED to READY transition.

I ran into an issue with asfdemux when going through the PAUSED->READY->PAUSED states. On the second time I change the state to PAUSED, asfdemux would cause a segmentation fault due to demux->global_metadata being NULL at gstasfdemux.c:3521. Looking further into it, I saw that the transition from NULL->READY initialized this variable, but the reset that occurs on PAUSED->READY destroys it. I'm not at all familiar with this codebase, so I'm not confident that this doesn't have unwanted side effects.