Fix Coverity 1547856: memset() uses only the lowest byte of c

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from #22430)

(cherry picked from commit 0bf1814)

test/rsa_test.c

@@ -484,7 +484,7 @@ static int test_EVP_rsa_legacy_key(void)
     0x85, 0x20, 0x4f, 0x35, 0x02, 0xfa, 0xda, 0x14, 0x77, 0xfa, 0x08, 0x34,
     0x60, 0xc7, 0x93, 0x72, 0xdc, 0xc4, 0x18, 0x70, 0xc1 };
 
-    memset(msgbuf, 0xdeadbeef, 64);
+    memset(msgbuf, 0xef, 64);
 
     ret = (TEST_ptr((p = BN_bin2bn(p_data, sizeof(p_data), NULL)))
            && TEST_ptr((q = BN_bin2bn(q_data, sizeof(q_data), NULL)))