network: unset Network::manager when loading .network file fails

network: unset Network::manager when loading .network file fails

Otherwise, LIST_REMOVE() in network_free() fails.

This fixes the following assertion:
```
systemd-networkd[2595]: Bus bus-api-network: changing state UNSET → OPENING
systemd-networkd[2595]: Bus bus-api-network: changing state OPENING → AUTHENTICATING
systemd-networkd[2595]: timestamp of '/etc/systemd/network' changed
systemd-networkd[2595]: /etc/systemd/network/10-hoge.network:1: Invalid section header '[Network]Address=192.168.0.1'
systemd-networkd[2595]: /etc/systemd/network/10-hoge.network:1: Failed to parse file: Bad message
systemd-networkd[2595]: Assertion '*_head == _item' failed at ../../home/watanabe/git/systemd/src/network/networkd-network.c:378, function network_free(). Aborting.
valgrind[2595]: ==2595==
valgrind[2595]: ==2595== Process terminating with default action of signal 6 (SIGABRT): dumping core
valgrind[2595]: ==2595==    at 0x4BCA53F: raise (in /usr/lib64/libc-2.28.so)
valgrind[2595]: ==2595==    by 0x4BB4894: abort (in /usr/lib64/libc-2.28.so)
valgrind[2595]: ==2595==    by 0x4955F09: log_assert_failed_realm (log.c:795)
valgrind[2595]: ==2595==    by 0x417101: network_free (networkd-network.c:378)
valgrind[2595]: ==2595==    by 0x415E99: network_freep (networkd-network.h:282)
valgrind[2595]: ==2595==    by 0x416AB2: network_load_one (networkd-network.c:101)
valgrind[2595]: ==2595==    by 0x416C39: network_load (networkd-network.c:293)
valgrind[2595]: ==2595==    by 0x414031: manager_load_config (networkd-manager.c:1502)
valgrind[2595]: ==2595==    by 0x40B258: run (networkd.c:82)
valgrind[2595]: ==2595==    by 0x40B74A: main (networkd.c:117)
valgrind[2595]: ==2595==
valgrind[2595]: ==2595== HEAP SUMMARY:
valgrind[2595]: ==2595==     in use at exit: 32,621 bytes in 201 blocks
valgrind[2595]: ==2595==   total heap usage: 746 allocs, 545 frees, 241,027 bytes allocated
valgrind[2595]: ==2595==
valgrind[2595]: ==2595== LEAK SUMMARY:
valgrind[2595]: ==2595==    definitely lost: 0 bytes in 0 blocks
valgrind[2595]: ==2595==    indirectly lost: 0 bytes in 0 blocks
valgrind[2595]: ==2595==      possibly lost: 0 bytes in 0 blocks
valgrind[2595]: ==2595==    still reachable: 32,621 bytes in 201 blocks
valgrind[2595]: ==2595==         suppressed: 0 bytes in 0 blocks
valgrind[2595]: ==2595== Reachable blocks (those to which a pointer was found) are not shown.
valgrind[2595]: ==2595== To see them, rerun with: --leak-check=full --show-leak-kinds=all
valgrind[2595]: ==2595==
valgrind[2595]: ==2595== For counts of detected and suppressed errors, rerun with: -v
valgrind[2595]: ==2595== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
systemd-coredump[2600]: Process 2595 (memcheck-amd64-) of user 192 dumped core.
```