deflate: avoid use of uninitialized variable

(Note emit_match() doesn't currently use the value at all.)

Fixes madler#4

deflate_medium.c

@@ -176,7 +176,7 @@ block_state deflate_medium(deflate_state *s, int flush)
     memset(&next_match, 0, sizeof(struct match));
 
     for (;;) {
-        IPos hash_head;       /* head of the hash chain */
+        IPos hash_head = 0;   /* head of the hash chain */
         int bflush;           /* set if current block must be flushed */
 
         /* Make sure that we always have enough lookahead, except