[filebuf] Add missing const to pointer variables

The fourth parameter of `codecvt::out` and `codecvt::in` are
`const C*&`, but _Effects_ use `C*` variables, which makes the call
ill-formed.

The intent seems sufficient clear and we should probably use `const C*`
variables in these places.

Author: frederick-vs-ja

source/iostreams.tex

@@ -11431,10 +11431,10 @@
 and then as if by doing:
 
 \begin{codeblock}
-char   extern_buf[XSIZE];
-char*  extern_end;
-charT  intern_buf[ISIZE];
-charT* intern_end;
+char         extern_buf[XSIZE];
+const char*  extern_end;
+charT        intern_buf[ISIZE];
+charT*       intern_end;
 codecvt_base::result r =
   a_codecvt.in(state, extern_buf, extern_buf+XSIZE, extern_end,
                intern_buf, intern_buf+ISIZE, intern_end);
@@ -11562,11 +11562,11 @@
 except that the behavior of ``consuming characters'' is performed by first
 converting as if by:
 \begin{codeblock}
-charT* b = pbase();
-charT* p = pptr();
-charT* end;
-char   xbuf[XSIZE];
-char*  xbuf_end;
+charT*       b = pbase();
+charT*       p = pptr();
+const charT* end;
+char         xbuf[XSIZE];
+char*        xbuf_end;
 codecvt_base::result r =
   a_codecvt.out(state, b, p, end, xbuf, xbuf+XSIZE, xbuf_end);
 \end{codeblock}