Treat an unexpected constant-sized VERSIONS cell as a PROTOCOL_WARN.

Treat an unexpected constant-sized VERSIONS cell as a PROTOCOL_WARN.

We previously used tor_fragile_assert() to declare that this case
could not happen: VERSIONS cells are always supposed to be
variable-sized, right?

This is incorrect, though.  On a v1 link protocol connection, all
cells are fixed-sized.  There aren't supposed to be any VERSIONS
cells with this version of the protocol, but apparently, somebody
was messing up.  (The v1 link protocol is obsolete, so probably the
implementer responsible didn't mean to be using it.)

Fixes bug 31107.  Bugfix on 0.2.4.4-alpha, when we introduced a
tor_fragile_assert() for this case.