Handle 0 return values from DH key computations as errors

Returned 0 from ossl_dh_compute_key(), DH_compute_key_padded() and
DH_compute_key() needs to be treated as an error.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27673)

(cherry picked from commit 1c1ce2a6ee)

crypto/dh/dh_pmeth.c

@@ -424,7 +424,7 @@ static int pkey_dh_derive(EVP_PKEY_CTX *ctx, unsigned char *key,
             ret = DH_compute_key_padded(key, dhpubbn, dh);
         else
             ret = DH_compute_key(key, dhpubbn, dh);
-        if (ret < 0)
+        if (ret <= 0)
             return ret;
         *keylen = ret;
         return 1;

test/dhtest.c

@@ -208,17 +208,17 @@ static int dh_test(void)
 
     alen = DH_size(a);
     if (!TEST_ptr(abuf = OPENSSL_malloc(alen))
-            || !TEST_true((aout = DH_compute_key(abuf, bpub_key, a)) != -1))
+            || !TEST_int_gt((aout = DH_compute_key(abuf, bpub_key, a)), 0))
         goto err3;
 
     blen = DH_size(b);
     if (!TEST_ptr(bbuf = OPENSSL_malloc(blen))
-            || !TEST_true((bout = DH_compute_key(bbuf, apub_key, b)) != -1))
+            || !TEST_int_gt((bout = DH_compute_key(bbuf, apub_key, b)), 0))
         goto err3;
 
     clen = DH_size(c);
     if (!TEST_ptr(cbuf = OPENSSL_malloc(clen))
-            || !TEST_true((cout = DH_compute_key(cbuf, apub_key, c)) != -1))
+            || !TEST_int_gt((cout = DH_compute_key(cbuf, apub_key, c)), 0))
         goto err3;
 
     if (!TEST_true(aout >= 20)
@@ -694,12 +694,12 @@ static int rfc7919_test(void)
 
     alen = DH_size(a);
     if (!TEST_int_gt(alen, 0) || !TEST_ptr(abuf = OPENSSL_malloc(alen))
-            || !TEST_true((aout = DH_compute_key(abuf, bpub_key, a)) != -1))
+            || !TEST_int_gt((aout = DH_compute_key(abuf, bpub_key, a)), 0))
         goto err;
 
     blen = DH_size(b);
     if (!TEST_int_gt(blen, 0) || !TEST_ptr(bbuf = OPENSSL_malloc(blen))
-            || !TEST_true((bout = DH_compute_key(bbuf, apub_key, b)) != -1))
+            || !TEST_int_gt((bout = DH_compute_key(bbuf, apub_key, b)), 0))
         goto err;
 
     if (!TEST_true(aout >= 20)