get_key_material_https: removed bogus free() call

The get_key_material_https() function error code path had a bogus
free() call, either resulting in double-free or free() of undefined
pointer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Co-authored-by: Harry Sintonen <sintonen@iki.fi>
Signed-off-by: Harry Sintonen <sintonen@iki.fi>
Closes #13198