BUG/MEDIUM: sample/jwt: fix another instance of base64 error detection

This is the same as for commit 468c000db ("BUG/MEDIUM: jwt: fix base64
decoding error detection"), but for function sample_conv_jwt_member_query()
that is used by sample converters jwt_header_query() and jwt_payload_query().
Thanks to Tim for the report. No backport is needed.
1 file changed