Fix ASSERTs that appears when enabling asserts in net buf.
Asserts are:
- Pulling from net buf before any data has been added.
- Pulling more data than has been allocated for the buf.
Fix warning:
W: You have 1 IPv4 net_if addresses but 2 network interfaces
W: Consider increasing CONFIG_NET_IF_MAX_IPV4_COUNT value.
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>