Bluetooth: Fix wrong set of skb fragments

If alloc() fails we let the frags linked list with garbage value (the
err ptr value) in its last element.

Reported-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>