compile error with undeclared variable in strrncmp

[Joungkyun]

compile error with follow messages:

libstring.c:1600:23: error: 'a' undeclared (first use in this function)
  s1len = safe_strlen (a);
                       ^
libstring.c:1600:23: note: each undeclared identifier is reported only once for each function it appears in
  s1len = safe_strlen (a);
                       ^
libstring.c:1601:23: error: 'b' undeclared (first use in this function)
  s2len = safe_strlen (b);

This bug does not affect 1.0.0 and before and is related to 01fe66e.