context: Don't leak EC_KEY in set_curve()

SSL_CTX_set_tmp_ecdh() takes a reference to the provided key.

==8323== 1,044 (56 direct, 988 indirect) bytes in 1 blocks are definitely lost in loss record 611 of 631
==8323==    at 0x4C2935B: malloc (vg_replace_malloc.c:270)
==8323==    by 0x5E05D9F: CRYPTO_malloc (mem.c:308)
==8323==    by 0x5E59859: EC_KEY_new (ec_key.c:75)
==8323==    by 0x5E59974: EC_KEY_new_by_curve_name (ec_key.c:96)
==8323==    by 0x4E395A7: set_curve (context.c:261)
...
ssl.lua: Comment subtle DH/ECDH ordering caveat