def_load_bio(): Free |biosk| more carefully

If there's anything in the |biosk| stack, the first element is always
the input BIO.  It should never be freed in this function, so we must
take careful steps not to do so inadvertently when freeing the stack.
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6769)

crypto/conf/conf_def.c

@@ -424,12 +424,26 @@ static int def_load_bio(CONF *conf, BIO *in, long *line)
     }
     BUF_MEM_free(buff);
     OPENSSL_free(section);
-    sk_BIO_pop_free(biosk, BIO_vfree);
+    /*
+     * No need to pop, since we only get here if the stack is empty.
+     * If this causes a BIO leak, THE ISSUE IS SOMEWHERE ELSE!
+     */
+    sk_BIO_free(biosk);
     return 1;
  err:
     BUF_MEM_free(buff);
     OPENSSL_free(section);
-    sk_BIO_pop_free(biosk, BIO_vfree);
+    /*
+     * Since |in| is the first element of the stack and should NOT be freed
+     * here, we cannot use sk_BIO_pop_free().  Instead, we pop and free one
+     * BIO at a time, making sure that the last one popped isn't.
+     */
+    while (sk_BIO_num(biosk) > 0) {
+        BIO *popped = sk_BIO_pop(biosk);
+        BIO_vfree(in);
+        in = popped;
+    }
+    sk_BIO_free(biosk);
 #ifndef OPENSSL_NO_POSIX_IO
     OPENSSL_free(dirpath);
     if (dirctx != NULL)