WPACKET: don't write DER length when we don't want to

With endfirst writing, it could be that we want to abandon any zero
length sub-packet.  That's what WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH
was supposed to make happen, but the DER length writing code didn't
look at that flag.  Now it does.
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/11703)

crypto/packet.c

@@ -265,7 +265,10 @@ static int wpacket_intern_close(WPACKET *pkt, WPACKET_SUB *sub, int doclose)
                 && !put_value(&buf[sub->packet_len], packlen,
                               sub->lenbytes))
             return 0;
-    } else if (pkt->endfirst && sub->parent != NULL) {
+    } else if (pkt->endfirst && sub->parent != NULL
+               && (packlen != 0
+                   || (sub->flags
+                       & WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH) == 0)) {
         size_t tmplen = packlen;
         size_t numlenbytes = 1;
 

test/wpackettest.c

@@ -360,6 +360,8 @@ static int test_WPACKET_init_der(void)
     unsigned char testdata[] = { 0x00, 0x01, 0x02, 0x03 };
     unsigned char testdata2[259]  = { 0x82, 0x01, 0x00 };
     size_t written[2];
+    size_t size1, size2;
+    int flags = WPACKET_FLAGS_ABANDON_ON_ZERO_LENGTH;
     int i;
 
     /* Test initialising for writing DER */
@@ -370,6 +372,13 @@ static int test_WPACKET_init_der(void)
             || !TEST_true(WPACKET_memcpy(&pkt, testdata, sizeof(testdata)))
             || !TEST_true(WPACKET_close(&pkt))
             || !TEST_true(WPACKET_put_bytes_u8(&pkt, 0xfc))
+            /* this sub-packet is empty, and should render zero bytes */
+            || (!TEST_true(WPACKET_start_sub_packet(&pkt))
+                || !TEST_true(WPACKET_set_flags(&pkt, flags))
+                || !TEST_true(WPACKET_get_total_written(&pkt, &size1))
+                || !TEST_true(WPACKET_close(&pkt))
+                || !TEST_true(WPACKET_get_total_written(&pkt, &size2))
+                || !TEST_size_t_eq(size1, size2))
             || !TEST_true(WPACKET_finish(&pkt))
             || !TEST_true(WPACKET_get_total_written(&pkt, &written[0]))
             || !TEST_mem_eq(WPACKET_get_curr(&pkt), written[0], simpleder,