res_rtp_asterisk: Don't leak temporary key when enabling PFS.

A change recently went in which enabled perfect forward secrecy for
DTLS in res_rtp_asterisk. This was accomplished two different ways
depending on the availability of a feature in OpenSSL. The fallback
method created a temporary instance of a key but did not free it.
This change fixes that.

ASTERISK-25265

Change-Id: Iadc031b67a91410bbefb17ffb4218d615d051396