samples: net: Fix incorrect error check in echo_server
samples: net: Fix incorrect error check in echo_server

The logic for error checking after net_pkt_pull was inverted -
build_pkt_reply would exit in case net_pkt_pull succeeded. In result no
responses were sent by echo_server.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

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