core/net/ip: Prevent (tiny) buffer overflow in resolv_found()

An off-by-one error in resolv_found() could make an strncat() call
overflow by the terminating null byte.

When building with Clang the following warning was shown:

../../../core/net/ip/resolv.c:1458:17: warning: the value of the
      size argument in 'strncat' is too large, might lead to a
      buffer overflow [-Wstrncat-size]
                sizeof(resolv_hostname) - strlen(resolv_hostname));
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../core/net/ip/resolv.c:1458:17: note: change the argument to
      be the free space in the destination buffer minus the
      terminating null byte
                sizeof(resolv_hostname) - strlen(resolv_hostname));
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                sizeof(resolv_hostname) - strlen(resolv_hostname) - 1

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #796 from gebart/core-net-ip-buffer-overflow-fix

core/net/ip: Prevent (tiny) buffer overflow in resolv_found()