Fix for systems with unsigned char

bzlib.rs

@@ -1788,7 +1788,7 @@ pub unsafe extern "C" fn BZ2_bzWrite(
     (*bzf).strm.next_in = buf as *mut libc::c_char;
     loop {
         (*bzf).strm.avail_out = 5000 as libc::c_int as libc::c_uint;
-        (*bzf).strm.next_out = ((*bzf).buf).as_mut_ptr();
+        (*bzf).strm.next_out = ((*bzf).buf).as_mut_ptr().cast::<c_char>();
         ret = BZ2_bzCompress(&mut (*bzf).strm, 0 as libc::c_int);
         if ret != 1 as libc::c_int {
             if !bzerror.is_null() {
@@ -1902,7 +1902,7 @@ pub unsafe extern "C" fn BZ2_bzWriteClose64(
     if abandon == 0 && (*bzf).lastErr == 0 as libc::c_int {
         loop {
             (*bzf).strm.avail_out = 5000 as libc::c_int as libc::c_uint;
-            (*bzf).strm.next_out = ((*bzf).buf).as_mut_ptr();
+            (*bzf).strm.next_out = ((*bzf).buf).as_mut_ptr().cast::<c_char>();
             ret = BZ2_bzCompress(&mut (*bzf).strm, 2 as libc::c_int);
             if ret != 3 as libc::c_int && ret != 4 as libc::c_int {
                 if !bzerror.is_null() {