btree: catch NULL value before it does harm

Storing NULL values in the btree is illegal and can lead to memory
corruption and possible other fun as well.  Catch it on insert, instead
of waiting for the inevitable.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>