deflate_medium: avoid emitting a suboptimal literal in the restart case

When we load new data into the window, we invalidate the next match, in
case the match would improve. In this case, the hash has already been
updated with this data, so when we look for a new match it will point
it back at itself. As a result, a literal is generated even when a
better match is available.

This avoids that by catching this case and ensuring we're looking at the
past.

Author: jtkukunas

deflate_medium.c

@@ -216,6 +216,9 @@ block_state deflate_medium(deflate_state *s, int flush)
             if (s->lookahead >= MIN_MATCH) {
                 INSERT_STRING(s, s->strstart, hash_head);
             }
+
+            if (hash_head && hash_head == s->strstart)
+                hash_head--;
 
             /* set up the initial match to be a 1 byte literal */
             current_match.match_start = 0;
@@ -249,6 +252,9 @@ block_state deflate_medium(deflate_state *s, int flush)
         if (s->lookahead > MIN_LOOKAHEAD) {
             s->strstart = current_match.strstart + current_match.match_length;
             INSERT_STRING(s, s->strstart, hash_head);
+
+            if (hash_head && hash_head == s->strstart)
+                hash_head--;
 
             /* set up the initial match to be a 1 byte literal */
             next_match.match_start = 0;