Fix `mosquitto_passwd -U` backup file creation.

Closes #2873.

ChangeLog.txt

@@ -5,6 +5,9 @@ Broker:
 - Fix `max_queued_message 0` stopping clients from receiving messages.
   Closes #2879.
 
+Apps:
+- Fix `mosquitto_passwd -U` backup file creation. Closes #2873.
+
 
 2.0.16 - 2023-08-16
 ===================

apps/mosquitto_passwd/mosquitto_passwd.c

@@ -629,13 +629,14 @@ int main(int argc, char *argv[])
 			return 1;
 		}
 
-		backup_file = malloc((size_t)strlen(password_file)+strlen(".backup.XXXXXX"));
+		size_t len = strlen(password_file) + strlen(".backup.XXXXXX") + 1;
+		backup_file = malloc(len);
 		if(!backup_file){
 			fprintf(stderr, "Error: Out of memory.\n");
 			free(password_file);
 			return 1;
 		}
-		snprintf(backup_file, strlen(password_file)+5, "%s.backup.XXXXXX", password_file);
+		snprintf(backup_file, len, "%s.backup.XXXXXX", password_file);
 		free(password_file);
 		password_file = NULL;