listen: avoid invalid memory access

listen: avoid invalid memory access

Fixes the following memory access error spotted by valgrind:

    Invalid read of size 4
       at 0x10D6D3: uh_socket_bind (listen.c:192)
       by 0x10C830: add_listener_arg (main.c:128)
       by 0x10C830: main (main.c:325)
     Address 0x4aa1160 is 0 bytes after a block of size 64 alloc'd
       at 0x483877F: malloc (vg_replace_malloc.c:307)
       by 0x49ACAC5: gaih_inet.constprop.0 (getaddrinfo.c:1058)
       by 0x49AE224: getaddrinfo (getaddrinfo.c:2256)
       by 0x10D590: uh_socket_bind (listen.c:145)
       by 0x10C830: add_listener_arg (main.c:128)
       by 0x10C830: main (main.c:325)

Signed-off-by: Jo-Philipp Wich <jo@mein.io>