Merge branch 'isdn-null-deref'

Karsten Keil says:

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Fix potential NULL pointer access and memory leak in ISDN layer2 functions

Insu Yun did brinup the issue with not checking the skb_clone() return
value in the layer2 I-frame ull functions.
This series fix the issue in a way which avoid protocol violations/data loss
on a temporary memory shortage.
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Signed-off-by: David S. Miller <davem@davemloft.net>