Memory leak on plugin load failure

[ossama-othman]

A quick leak check through valgrind reveals a small memory leak at process exit on plugin load failure:

==11756== 40 bytes in 1 blocks are definitely lost in loss record 19 of 25
==11756==    at 0x4839809: malloc (vg_replace_malloc.c:307)
==11756==    by 0x48608FD: l_malloc (util.c:62)
==11756==    by 0x4865D26: watch_add (main.c:144)
==11756==    by 0x4867158: l_io_new (io.c:191)
==11756==    by 0x486BC54: l_genl_new (genl.c:1140)
==11756==    by 0x10D59F: mptcpd_pm_create (path_manager.c:1166)
==11756==    by 0x10B62E: main (mptcpd.c:58)

Originally posted by @ossama-othman in https://github.com/intel/mptcpd/pull/98#discussion_r557843831