Duplicate a substring
char *SLmake_nstring (char *s, unsigned int n)
This function is like SLmake_nstring except that it creates a
null terminated string formed from the first n characters of
s. Upon failure, it returns NULL, otherwise it returns
the new string. When nolonger needed, the returned string should be
freed with either free or SLfree.
SLmake_nstring, SLfree, SLang_create_nslstring