windows: fix issues detected by clang-tidy, and some more

- digest_sspi: memory leak.
- digest_sspi: free buffers on `calloc()` fail.
  (not detected by clang-tidy)
- schannel_verify: avoid a `NULL` `alt_name_info`.
- schannel: fix potential `NULL` deref for `backend->cred`.
- schannel: fix uninitialized result value.
  Follow-up to 7f4c358541 #3197
- schannel: drop unused assigment.
- tool_doswin: drop unused assigment.
- testutil: fix memory leak on error.
- testutil: fix memory leak on non-error.
  (not detected by clang-tidy)

Cherry-picked from #16764
Closes #16777

lib/vauth/digest_sspi.c

@@ -582,8 +582,12 @@ CURLcode Curl_auth_create_digest_http_message(struct Curl_easy *data,
 
     /* Allocate our new context handle */
     digest->http_context = calloc(1, sizeof(CtxtHandle));
-    if(!digest->http_context)
+    if(!digest->http_context) {
+      curlx_unicodefree(spn);
+      Curl_sspi_free_identity(p_identity);
+      free(output_token);
       return CURLE_OUT_OF_MEMORY;
+    }
 
     /* Generate our response message */
     status = Curl_pSecFn->InitializeSecurityContext(&credentials, NULL,