dnsproxy: Fix wrong domain len causing crash in memmove

dnsproxy: Fix wrong domain len causing crash in memmove

If host_len is 0, strlen(*ptr) will return 0 as well and domain_len will
have the wrong value (-1). This causes connman to crash with the
following backtrace:

connmand[13361]: ++++++++ backtrace ++++++++
connmand[13361]: #0  0x7f33a5e49140 in /lib/libpthread.so.0
connmand[13361]: #1  0x7f33a509ed80 in /lib/libc.so.6
connmand[13361]: #2  0x4671a8 in forward_dns_reply() at src/dnsproxy.c:1568
connmand[13361]: #3  0x46725a in udp_server_event() at src/dnsproxy.c:1706
connmand[13361]: #4  0x7f33a62e14a5 in /usr/lib/libglib-2.0.so.0
connmand[13361]: #5  0x7f33a62e17d8 in /usr/lib/libglib-2.0.so.0
connmand[13361]: #6  0x7f33a62e1bd2 in /usr/lib/libglib-2.0.so.0
connmand[13361]: #7  0x40dfa1 in main() at src/main.c:510
connmand[13361]: #8  0x7f33a4f8e455 in /lib/libc.so.6
connmand[13361]: +++++++++++++++++++++++++++

To reproduce this issue, we need to have a domain name set and make a
query for "". The easiest way to do so is to use the (wrong) git command
below:

	git push :your.host.name

Make sure we calculate domain_len the right way. By guaranteeing
host_len and domain_len are >= we can also use memcpy() instead of
memmove().