BUG/MINOR: quic: do not crash on CRYPTO ncbuf alloc failure

BUG/MINOR: quic: do not crash on CRYPTO ncbuf alloc failure

To handle out-of-order received CRYPTO frames, a ncbuf instance is
allocated. This is done via the helper quic_get_ncbuf().

Buffer allocation was improperly checked. In case b_alloc() fails, it
crashes due to a BUG_ON(). Fix this by removing it. The function now
returns NULL on allocation failure, which is already properly handled in
its caller qc_handle_crypto_frm().

This should fix the last reported crash from github issue #2935.

This must be backported up to 2.6.