Remove redundant memset call following palloc0

This is a follow-up commit to ca7f8e2 which removed the allocation
abstraction from pgcrypto and replaced px_alloc + memset calls with
palloc0 calls. The particular memset in this commit was missed in
that work though.

Author: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CALNJ-vT5qRucrFMPSzQyAWods1b4MnNPG-M=_ZUzh1SoTh0vNw@mail.gmail.com

contrib/pgcrypto/px.c

@@ -203,7 +203,6 @@ combo_init(PX_Combo *cx, const uint8 *key, unsigned klen,
 	if (klen > ks)
 		klen = ks;
 	keybuf = palloc0(ks);
-	memset(keybuf, 0, ks);
 	memcpy(keybuf, key, klen);
 
 	err = px_cipher_init(c, keybuf, klen, ivbuf);