Fix warning reported by nelgin.

src/sbbs3/answer.cpp

@@ -78,9 +78,9 @@ check_pubkey(scfg_t *cfg, ushort unum, char *pkey, size_t pksz)
 						char pk[2048];
 						int pklen;
 						pklen = b64_decode(pk, sizeof(pk), tok, 0);
-						if (pklen != -1) {
+						if (pklen > 0) {
 							if ((pksz - 4) == pklen) {
-								if (memcmp(&pkey[4], pk, pklen) == 0) {
+								if (memcmp(&pkey[4], pk, (unsigned)pklen) == 0) {
 									fclose(sshkeys);
 									return true;
 								}