Fix #18

Following PR #19

src/ttail_init.c

 
 int ttail_set_fmt(ttail_t* t, const char* fmt)
 {
-	size_t len;
 	if(t->flag & TTAIL_FLAG_FORMAT)
 	{
 		return -1;
 	}
-	len = strlen(fmt);
-	t->fmt = malloc(sizeof(char)*(len+1));
+	t->fmt = strdup(fmt);
 	if(!t->fmt)
 	{
 		return -1;
 	}
-	strncpy(t->fmt, fmt, len);
-	t->fmt[len] = '\0';
 	t->flag |= TTAIL_FLAG_FORMAT;
 	return 0;
 }