Fix "get_key_index" to fit caller's expectations
    
    The clients of "get_key_index" expect index of key in matching case
    otherwise -1. But instead of this function returns the "undefined"
    value.
    
    Fixes: https://pagure.io/freeipa/issue/7678
    Reviewed-By: Serhii Tsymbaliuk <stsymbal@redhat.com>