Fix a couple security nits from codeql

See:
- https://github.com/brndnmtthws/conky/security/code-scanning/1
- https://github.com/brndnmtthws/conky/security/code-scanning/2
- https://github.com/brndnmtthws/conky/security/code-scanning/3

src/diskio.cc

@@ -49,7 +49,7 @@ void clear_diskio_stats() {
     cur = stats.next;
     stats.next = stats.next->next;
     free_and_zero(cur->dev);
-    free(cur);
+    delete cur;
   }
 }
 

src/imlib2.cc

@@ -107,7 +107,7 @@ void cimlib_cleanup() {
   while (cur != nullptr) {
     last = cur;
     cur = last->next;
-    free(last);
+    delete last;
   }
   image_list_start = image_list_end = nullptr;
 }

src/mail.cc

@@ -249,8 +249,7 @@ static void update_mail_count(struct local_mail_s *mail) {
       /* . and .. are skipped */
       if (dirent->d_name[0] != '.') {
         mail->mail_count++;
-        mailflags = static_cast<char *>(
+        mailflags = strdup(dirent->d_name);
-            malloc(sizeof(char) * strlen(strrchr(dirent->d_name, ','))));
         if (mailflags == nullptr) {
           NORM_ERR("malloc");
           return;