l-bfgs-s `f` & `g` might be mutable

[colinfang]

According to the fortran code

c     f is a double precision variable.
c       On first entry f is unspecified.
c       On final exit f is the value of the function at x.
c
c     g is a double precision array of dimension n.
c       On first entry g is unspecified.
c       On final exit g is the value of the gradient at x.

It is not clear whether f & g are not modified in the subroutine.