fftools/textformat: Apply quality improvements

Perform multiple improvements to increase code robustness.
In particular:
- favor unsigned counters for loops
- add missing checks
- avoid possible leaks
- move variable declarations to inner scopes when feasible
- provide explicit type-casting when needed

Signed-off-by: softworkz <softworkz@hotmail.com>

fftools/textformat/tf_xml.c

@@ -18,10 +18,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include <limits.h>
-#include <stdarg.h>
 #include <stdint.h>
-#include <stdio.h>
 #include <string.h>
 
 #include "avtextformat.h"