Fix a gcc 11.2.0 warning

gcc 11.2.0 is the default on Ubuntu 21.10. It emits a (spurious) warning
when compiling test/packettest.c, which causes --strict-warnings builds
to fail. A simple fix avoids the warning.
Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16887)

test/packettest.c

@@ -302,7 +302,7 @@ static int test_PACKET_forward(void)
 
 static int test_PACKET_buf_init(void)
 {
-    unsigned char buf1[BUF_LEN];
+    unsigned char buf1[BUF_LEN] = { 0 };
     PACKET pkt;
 
     /* Also tests PACKET_remaining() */