topdown/tokens: protect against nistec panics (#5214)

With Golang 1.19, the code used in crypto/elliptic has switched to nistec, and
this introduced new panics where there hadn't been any before.

To play along with this, we're adding defer/recover constructs. It's only about
bad inputs; none of those offending points could ever be valid.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>