examples: fix more potential resource leaks, and more

Also:
- delete dead code.
- sync `http2-download.c` and `http2-upload.c` sources.
- simplessl: fix constant expression.
- simplessl: avoid `expression is constant` VS2010 warning, drop pragma.
- replace large stack buffers with dynamic allocation.
- http2-download: fix to fill transfer number.

Some of these were pointed out by TIOBE scanner via Coverity 2025.3.0.

Closes #19292

docs/examples/http2-serverpush.c

@@ -225,14 +225,15 @@ int main(int argc, char *argv[])
 
   /* init a multi stack */
   multi_handle = curl_multi_init();
+  if(!multi_handle)
+    goto error;
 
   easy = curl_easy_init();
 
   /* set options */
-  if(setup(easy, url)) {
+  if(!easy || setup(easy, url)) {
     fprintf(stderr, "failed\n");
-    curl_global_cleanup();
-    return 1;
+    goto error;
   }
 
   curl_multi_setopt(multi_handle, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);
@@ -272,7 +273,11 @@ int main(int argc, char *argv[])
 
   } while(transfers); /* as long as we have transfers going */
 
-  curl_multi_cleanup(multi_handle);
+error:
+
+  if(multi_handle)
+    curl_multi_cleanup(multi_handle);
+
   curl_global_cleanup();
 
   fclose(out_download);