urisourcebin: Fix crash during EOS handling

Submitted by Seungha Yang

Description

Slot has two cases to receive fake EOS.
a) actual EOS from demuxer's pad
b) demuxer's pad-removed.
When a) happened, slot is still marked as linked.
And when b) happend, slot will be marked as unlinked.

Sometimes, following sequence is possible, and it causes crash

demuxer pushed EOS (fake-eos-A)
Then, the demuxer's pad was removed (fake-eos-A)
slot's srcpad got "fake-eos-A" but it was already marked as unlinked by
pad-removed. So, urisourcebin will throw the slot to threadpool to remove it.
"fake-eos-B" reached to slot's src pad and trying to free the slot again.