fix 'format_small_integer' return value

5e39c2a made a fix for format_integer, but format_small_integer has
the same problem, and returns an incorrect pointer. Apply a similar
fix to format_small_integer.
Merge pull request #5 from swiftserve/format_small_integer_bug_fix

Fix 'format_small_integer' return value.
Merge remote-tracking branch 'upstream/master'