p2p: Fix memory leak in p2p_listener_event

p2p: Fix memory leak in p2p_listener_event

g_io_add_watch keeps own reference to iochannel so local reference
should be dropped before leaving.

882 (840 direct, 42 indirect) bytes in 7 blocks are definitely lost in loss record 177 of 192
   at 0x4C2B6CD: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4E7FA78: g_malloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.3)
   by 0x4EB66F4: g_io_channel_unix_new (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.3)
   by 0x416C6F: p2p_listener_event (p2p.c:187)
   by 0x4E79D52: g_main_context_dispatch (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.3)
   by 0x4E7A09F: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.3)
   by 0x4E7A499: g_main_loop_run (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.3200.3)
   by 0x408DDE: main (main.c:221)