use utf8_length() instead of local function, isValidUtf8()

utf8_length() from libutils returns -1 when source not contains
valid sequence for UTF-8. Fixed to use it and removed the local
function isValidUtf8().

Change-Id: If2834ce1d1ae07fd8526ce8bc5df3fd3f44e85c8
Signed-off-by: Homin Lee <suapapa@insignal.co.kr>
1 file changed