channel: Fix use after free in channel_do_open_actions()

Fortunately, our tor_free() is setting the variable to NULL after so we were in a situation where NULL was always used instead of the transport name.

This first appeared in 894ff2dc and results in basically no bridge with a transport being able to use DoS defenses.

Fixes #40345 (closed)

Signed-off-by: David Goulet dgoulet@torproject.org