docs: make 5 example snippets compile cleanly with clang

verify-examples.pl invokes 'gcc', which on macOS is clang. With
-Wall -Wextra... -Werror -pedantic, clang rejects two patterns that
GNU gcc accepts:

- passing a char * to %p without a cast (-Wformat-pedantic): the C
  standard says %p expects void *, so cast explicitly in the
  CURLOPT_PROGRESSDATA, CURLOPT_PROGRESSFUNCTION, CURLOPT_XFERINFODATA
  and CURLOPT_XFERINFOFUNCTION examples

- returning CURLcode result from main() when curl_easy_init() returned
  NULL leaves it uninitialized (-Wsometimes-uninitialized) in the
  CURLINFO_TLS_SSL_PTR example: initialize it to CURLE_OK like other
  man pages already do

Closes #22638

docs/libcurl/opts/CURLINFO_TLS_SSL_PTR.md

@@ -142,7 +142,7 @@ static size_t wf(char *ptr, size_t size, size_t nmemb, void *stream)
 
 int main(int argc, char *argv[])
 {
-  CURLcode result;
+  CURLcode result = CURLE_OK;
   curl = curl_easy_init();
   if(curl) {
     curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

docs/libcurl/opts/CURLOPT_PROGRESSDATA.md

@@ -50,7 +50,7 @@ static int progress_callback(void *clientp,
                              double ulnow)
 {
   struct progress *memory = clientp;
-  printf("private: %p\n", memory->private);
+  printf("private: %p\n", (void *)memory->private);
 
   /* use the values */
 

docs/libcurl/opts/CURLOPT_PROGRESSFUNCTION.md

@@ -96,7 +96,7 @@ static int progress_callback(void *clientp,
                              double ulnow)
 {
   struct progress *memory = clientp;
-  printf("private: %p\n", memory->private);
+  printf("private: %p\n", (void *)memory->private);
 
   /* use the values */
 

docs/libcurl/opts/CURLOPT_XFERINFODATA.md

@@ -53,7 +53,7 @@ static int progress_cb(void *clientp,
                        curl_off_t ulnow)
 {
   struct progress *memory = clientp;
-  printf("private ptr: %p\n", memory->private);
+  printf("private ptr: %p\n", (void *)memory->private);
   /* use the values */
 
   return 0; /* all is good */

docs/libcurl/opts/CURLOPT_XFERINFOFUNCTION.md

@@ -92,7 +92,7 @@ static int xferinfo_callback(void *clientp,
                              curl_off_t ulnow)
 {
   struct progress *memory = clientp;
-  printf("my ptr: %p\n", memory->private);
+  printf("my ptr: %p\n", (void *)memory->private);
 
   /* use the values */