connection: fix asan report on sendmsg call

Initialiaze the entire msghdr struct to 0 before use.

Example of the report fixed with this change:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==20035==ERROR: AddressSanitizer: SEGV on unknown address 0x2dad4dbffffa0d (pc 0x0055555c7488 bp 0x007fffffc760 sp 0x007fffffc760 T0)
==20035==The signal is caused by a READ memory access.
    #0 0x55555c7488 in read_msghdr(void*, __sanitizer::__sanitizer_msghdr*, long) (/home/ftrvx/w/_/uxn/bin/uxnemu+0x77488)
    #1 0x55555c810c in sendmsg (/home/ftrvx/w/_/uxn/bin/uxnemu+0x7810c)
    #2 0x7ff7f2db20 in wl_connection_flush /home/ftrvx/q/wayland/build/../src/connection.c:315:10
    #3 0x7ff7f2d014 in wl_display_flush /home/ftrvx/q/wayland/build/../src/wayland-client.c:2154:9
    #4 0x7ff7e80bc0  (/lib/libSDL2-2.0.so.0+0x104bc0)
    #5 0x7ff7e523b0  (/lib/libSDL2-2.0.so.0+0xd63b0)
    #6 0x7ff7e534e4  (/lib/libSDL2-2.0.so.0+0xd74e4)
    #7 0x7ff7e535e8  (/lib/libSDL2-2.0.so.0+0xd75e8)
    #8 0x7ff7daad54  (/lib/libSDL2-2.0.so.0+0x2ed54)
    #9 0x7ff7dab130  (/lib/libSDL2-2.0.so.0+0x2f130)
    #10 0x555565bb40 in main /home/ftrvx/w/_/uxn/src/uxnemu.c:519:2
    #11 0x7ff7f62484 in libc_start_main_stage2 /builddir/musl-1.1.24/src/env/__libc_start_main.c:94:2

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/home/ftrvx/w/_/uxn/bin/uxnemu+0x77488) in read_msghdr(void*, __sanitizer::__sanitizer_msghdr*, long)
==20035==ABORTING

Signed-off-by: Sigrid Solveig Haflínudóttir <ftrvxmtrx@gmail.com>