Initialize zstream state so that cleanup doesn't dereference garbage pointer. (#141)

Author: samuel-williams-shopify

ext/zlib/zlib.c

@@ -651,6 +651,7 @@ zstream_init(struct zstream *z, const struct zstream_funcs *func)
 z->stream.avail_in=0;
 z->stream.next_out=Z_NULL;
 z->stream.avail_out=0;
+z->stream.state=Z_NULL;
 z->func=func;
 }
 

test/zlib/test_zlib.rb

@@ -1524,6 +1524,8 @@ def test_deflate_stream
 end
 
 deftest_gzip
+assert_raise(Zlib::StreamError){Zlib.gzip("foo",level: 100)}
+
 actual=Zlib.gzip("foo".freeze)
 actual[4,4]="\x00\x00\x00\x00"# replace mtime
 actual[9]="\xff"# replace OS