fix bug #207: avoid undefined integer overflow

zlib-ng used to fail when compiled with UBSan with this error:
deflate_slow.c:112:21: runtime error: unsigned integer overflow: 45871 - 45872 cannot be represented in type 'unsigned int'

The bug occurs in code added to zlib-ng under `#ifndef NOT_TWEAK_COMPILER`.
The original code of zlib contains a loop with two induction variables:

  s->prev_length -= 2;
  do {
      if (++s->strstart <= max_insert) {
          functable.insert_string(s, s->strstart, 1);
      }
  } while (--s->prev_length != 0);

The function insert_string is not executed when
  !(++s->strstart <= max_insert)
i.e., when
  !(s->strstart + 1 <= max_insert)
  !(s->strstart < max_insert)
  max_insert <= s->strstart

The function insert_string is executed when
  ++s->strstart <= max_insert
i.e., when
  s->strstart + 1 <= max_insert
  s->strstart < max_insert

The function is executed at most `max_insert - s->strstart` times, following the
exit condition of the do-while `(--s->prev_length != 0)`.  If the loop exits
after evaluating the exit condition once, the function is executed once
independently of `max_insert - s->strstart`.  The number of times the function
executes is the minimum between the number of iterations in the do-while loop
and `max_insert - s->strstart`.

The number of iterations of the loop is `mov_fwd = s->prev_length - 2`, and we
know that this is at least one as otherwise `--s->prev_length` would overflow.

The number of times the function insert_string is called is
  `min(mov_fwd, max_insert - s->strstart)`

1 files changed, 6 insertions, 4 deletions

diff --git a/deflate_slow.c b/deflate_slow.c
index 1763477..de53b2d 100644
--- a/deflate_slow.c
+++ b/deflate_slow.c
@@ -108,11 +108,13 @@ ZLIB_INTERNAL block_state deflate_slow(deflate_state *s, int flush) {
 #else
             {
                 unsigned int mov_fwd = s->prev_length - 2;
-                unsigned int insert_cnt = mov_fwd;
-                if (unlikely(insert_cnt > max_insert - s->strstart))
-                    insert_cnt = max_insert - s->strstart;
+                if (max_insert > s->strstart) {
+                    unsigned int insert_cnt = mov_fwd;
+                    if (unlikely(insert_cnt > max_insert - s->strstart))
+                        insert_cnt = max_insert - s->strstart;
 
-                functable.insert_string(s, s->strstart + 1, insert_cnt);
+                    functable.insert_string(s, s->strstart + 1, insert_cnt);
+                }
                 s->prev_length = 0;
                 s->match_available = 0;
                 s->match_length = MIN_MATCH-1;