Don't leak memory in the event of a failure in i2v_GENERAL_NAMES

i2v_GENERAL_NAMES call i2v_GENERAL_NAME repeatedly as required. Each
time i2v_GENERAL_NAME gets called it allocates adds data to the passed in
stack and then returns a pointer to the stack, or NULL on failure. If
the passed in stack is itself NULL then it allocates one.

i2v_GENERAL_NAMES was not correctly handling the case where a NULL gets
returned from i2v_GENERAL_NAME. If a stack had already been allocated then
it just leaked it.
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10300)

(cherry picked from commit 45b244620a74248b46ebe1c85e86437b9641447a)

crypto/x509v3/v3_alt.c

@@ -52,11 +52,24 @@ STACK_OF(CONF_VALUE) *i2v_GENERAL_NAMES(X509V3_EXT_METHOD *method,
 {
     int i;
     GENERAL_NAME *gen;
+    STACK_OF(CONF_VALUE) *tmpret = NULL, *origret = ret;
+
     for (i = 0; i < sk_GENERAL_NAME_num(gens); i++) {
         gen = sk_GENERAL_NAME_value(gens, i);
-        ret = i2v_GENERAL_NAME(method, gen, ret);
+        /*
+         * i2v_GENERAL_NAME allocates ret if it is NULL. If something goes
+         * wrong we need to free the stack - but only if it was empty when we
+         * originally entered this function.
+         */
+        tmpret = i2v_GENERAL_NAME(method, gen, ret);
+        if (tmpret == NULL) {
+            if (origret == NULL)
+                sk_CONF_VALUE_pop_free(ret, X509V3_conf_free);
+            return NULL;
+        }
+        ret = tmpret;
     }
-    if (!ret)
+    if (ret == NULL)
         return sk_CONF_VALUE_new_null();
     return ret;
 }