fix udp.srcaddr after connections realloc

[msaf1980]

Fix for #346

[msaf1980]

Trace with ASAN
==11648==ERROR: AddressSanitizer: heap-use-after-free on address 0x7f16b52e9811 at pc 0x00000040f9d6 bp 0x7f16b3ae39d0 sp 0x7f16b3ae39c0
WRITE of size 1 at 0x7f16b52e9811 thread T4
#0 0x40f9d5 in udpsockread /data/git/carbon-c-relay/dispatcher.c:163
#1 0x413154 in dispatch_connection /data/git/carbon-c-relay/dispatcher.c:859
#2 0x4152c4 in dispatch_runner /data/git/carbon-c-relay/dispatcher.c:1137
#3 0x7f16bcca8593 in start_thread (/lib64/libpthread.so.0+0x7593)
#4 0x7f16bc9dbe6e in clone (/lib64/libc.so.6+0xf9e6e)

0x7f16b52e9811 is located 17 bytes inside of 68247552-byte region [0x7f16b52e9800,0x7f16b93ff800)
freed by thread T1 here:
#0 0x7f16be05f448 in __interceptor_realloc (/lib64/libasan.so.5+0xef448)
#1 0x411126 in dispatch_addconnection /data/git/carbon-c-relay/dispatcher.c:600
#2 0x414dd0 in dispatch_runner /data/git/carbon-c-relay/dispatcher.c:1100
#3 0x7f16bcca8593 in start_thread (/lib64/libpthread.so.0+0x7593)

Realloc of connections is a root cause of segfault
https://github.com/grobian/carbon-c-relay/blob/master/dispatcher.c#L600