device: handle peer post config on blank line

We missed a function exit point. This was exacerbated by e3134bf
("device: defer state machine transitions until configuration is
complete"), but the bug existed prior. Minus provided the following
useful reproducer script:

    #!/usr/bin/env bash

    set -eux

    make wireguard-go || exit 125

    ip netns del test-ns || true
    ip netns add test-ns
    ip link add test-kernel type wireguard
    wg set test-kernel listen-port 0 private-key <(echo "QMCfZcp1KU27kEkpcMCgASEjDnDZDYsfMLHPed7+538=") peer "eDPZJMdfnb8ZcA/VSUnLZvLB2k8HVH12ufCGa7Z7rHI=" allowed-ips 10.51.234.10/32
    ip link set test-kernel netns test-ns up
    ip -n test-ns addr add 10.51.234.1/24 dev test-kernel
    port=$(ip netns exec test-ns wg show test-kernel listen-port)

    ip link del test-go || true
    ./wireguard-go test-go
    wg set test-go private-key <(echo "WBM7qimR3vFk1QtWNfH+F4ggy/hmO+5hfIHKxxI4nF4=") peer "+nj9Dkqpl4phsHo2dQliGm5aEiWJJgBtYKbh7XjeNjg=" allowed-ips 0.0.0.0/0 endpoint 127.0.0.1:$port
    ip addr add 10.51.234.10/24 dev test-go
    ip link set test-go up

    ping -c2 -W1 10.51.234.1

Reported-by: minus <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>

device/uapi.go

@@ -159,6 +159,7 @@ func (device *Device) IpcSetOperation(r io.Reader) (err error) {
 		line := scanner.Text()
 		if line == "" {
 			// Blank line means terminate operation.
+			peer.handlePostConfig()
 			return nil
 		}
 		parts := strings.Split(line, "=")