basic/log: force log_*_errno() to return negative

basic/log: force log_*_errno() to return negative

This silences some warnigns where gcc thinks that some variables are
unitialized. One particular case:

../src/journal/journald-server.c: In function 'ache_space_refresh':
../src/journal/journald-server.c:136:28: error: 'vfs_avail' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  136 |         uint64_t vfs_used, vfs_avail, avail;
      |                            ^~~~~~~~~
../src/journal/journald-server.c:136:18: error: 'vfs_used' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  136 |         uint64_t vfs_used, vfs_avail, avail;
      |                  ^~~~~~~~
cc1: all warnings being treated as errors

which is caused by

   d = opendir(path);
   if (!d)
           return log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_ERR,
                                 errno, "Failed to open %s: %m", path);
   if (fstatvfs(dirfd(d), &ss) < 0)
           return log_error_errno(errno, "Failed to fstatvfs(%s): %m", path);

For some reason on aarch64 gcc thinks we might return non-negative here. In
principle errno must be set in both cases, but it's hard to say for certain.
So let's make sure that our code flow is correct, even if somebody forgot to
set the global variable somewhere.