shared/hostname-setup: leave the terminator byte alone

gethostname(3) says it's unspecified whether the string is properly terminated
when the hostname is too long. We created a buffer with one extra byte, and it
seems the intent was to let that byte serve as terminator even if we get an
unterminated string from gethostname().

src/shared/hostname-setup.c

@@ -22,7 +22,7 @@ static int sethostname_idempotent_full(const char *s, bool really) {
 
         assert(s);
 
-        if (gethostname(buf, sizeof(buf)) < 0)
+        if (gethostname(buf, sizeof(buf) - 1) < 0)
                 return -errno;
 
         if (streq(buf, s))