v4l2videodec: ensure pool exists before orphaning it

v4l2videodec: ensure pool exists before orphaning it

In commit e2ff87732d0b ("v4l2videodec: support orphaning") support for
orphaning the capture buffer pool was added when the format is
renegotiated. However, the commit forgot to check that a pool existed
before doing this. This is needed because it's possible for the format
to be renegotiated before a capture pool is allocated, which would
result in trying to orphan a NULL pool and lead to a NULL pointer
dereference.

Fix this by checking a pool exists first. If the pool doesn't exist,
there are no buffers to be reclaimed, so skip the allocation query in
that case.