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/smtp-authzid.c

@@ -72,6 +72,7 @@ static size_t payload_source(char *ptr, size_t size, size_t nmemb, void *userp)
   struct upload_status *upload_ctx = (struct upload_status *)userp;
   const char *data;
   size_t room = size * nmemb;
+  size_t len;
 
   if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
     return 0;
@@ -79,17 +80,13 @@ static size_t payload_source(char *ptr, size_t size, size_t nmemb, void *userp)
 
   data = &payload_text[upload_ctx->bytes_read];
 
-  if(data) {
-    size_t len = strlen(data);
-    if(room < len)
-      len = room;
-    memcpy(ptr, data, len);
-    upload_ctx->bytes_read += len;
+  len = strlen(data);
+  if(room < len)
+    len = room;
+  memcpy(ptr, data, len);
+  upload_ctx->bytes_read += len;
 
-    return len;
-  }
-
-  return 0;
+  return len;
 }
 
 int main(void)