h264/h265parse: Negotiation could fail earlier

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#754127)

Description

In these parsers negotiation function, we do get_allowed_caps(). This this returns empty caps, we could fail immediately. Note that this may require a little more work because of the flushing special case. We need not to return not-negotiated if some element downstream fails due to FLOW_FLUSHING. Also, as this call is recursive, I'm unsure if we can check the peer pad flushing flag only as we usually do to handle this special case.