trace: fix out-of-bound memory access

When OSSL_trace_get_category_num() is called with an unknown category
name, it returns -1. This case needs to be considered in order to
avoid out-of-bound memory access to the `trace_channels` array.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8552)