slip: make slhc_free() silently accept an error pointer

This way, slhc_free() accepts what slhc_init() returns, whether that is
an error or not.

In particular, the pattern in sl_alloc_bufs() is

        slcomp = slhc_init(16, 16);
        ...
        slhc_free(slcomp);

for the error handling path, and rather than complicate that code, just
make it ok to always free what was returned by the init function.

That's what the code used to do before commit 4ab42d78 ("ppp, slip:
Validate VJ compression slot parameters completely") when slhc_init()
just returned NULL for the error case, with no actual indication of the
details of the error.

Reported-by:  <syzbot+45474c076a4927533d2e@syzkaller.appspotmail.com>
Fixes: 4ab42d78 ("ppp, slip: Validate VJ compression slot parameters completely")
Acked-by: Ben Hutchings <ben@decadent.org.uk>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>