BUG/MINOR: jwt: Copy input and parameters in dedicated buffers in jwt_verify converter

BUG/MINOR: jwt: Copy input and parameters in dedicated buffers in jwt_verify converter

When resolving variable values the temporary trash chunks are used so
when calling the 'jwt_verify' converter with two variable parameters
like in the following line, the input would be overwritten by the value
of the second parameter :
    var(txn.bearer),jwt_verify(txn.jwt_alg,txn.cert)
Copying the values into dedicated alloc'ed buffers prevents any new call
to get_trash_chunk from erasing the data we need in the converter.

This patch can be backported up to 2.8.