zlib: initialize git_zstream in git_deflate_init{,_gzip,_raw}

Clear the git_zstream variable at the start of git_deflate_init() etc.
so that callers don't have to do that.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2091,7 +2091,6 @@ static unsigned char *deflate_it(char *data,
 	unsigned char *deflated;
 	git_zstream stream;
 
-	memset(&stream, 0, sizeof(stream));
 	git_deflate_init(&stream, zlib_compression_level);
 	bound = git_deflate_bound(&stream, size);
 	deflated = xmalloc(bound);