t/helper/test-read-midx.c: plug memory leak when selecting layer

Though our 'read-midx' test tool is capable of printing information
about a single MIDX layer identified by its checksum, no caller in our
test suite exercises this path.

Unfortunately, there is a memory leak lurking in this (currently) unused
path that would otherwise be exposed by the following commit.

This occurs when providing a MIDX layer checksum other than the tip. As
we walk over the MIDX chain trying to find the matching layer, we drop
our reference to the top-most MIDX layer. Thus, our call to
'close_midx()' later on leaks memory between the top-most MIDX layer and
the MIDX layer immediately following the specified one.

Plug this leak by holding a reference to the tip of the MIDX chain, and
ensure that we call `close_midx()` before terminating the test tool.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ttaylorr

t/helper/test-read-midx.c

@@ -26,18 +26,22 @@ static int read_midx_file(const char *object_dir, const char *checksum,
 			  int show_objects)
 {
 	uint32_t i;
-	struct multi_pack_index *m;
+	struct multi_pack_index *m, *tip;
+	int ret = 0;
 
-	m = setup_midx(object_dir);
+	m = tip = setup_midx(object_dir);
 
 	if (!m)
 		return 1;
 
 	if (checksum) {
 		while (m && strcmp(get_midx_checksum(m), checksum))
 			m = m->base_midx;
-		if (!m)
-			return 1;
+		if (!m) {
+			ret = error(_("could not find MIDX with checksum %s"),
+				    checksum);
+			goto out;
+		}
 	}
 
 	printf("header: %08x %d %d %d %d\n",
@@ -82,9 +86,10 @@ static int read_midx_file(const char *object_dir, const char *checksum,
 		}
 	}
 
-	close_midx(m);
+out:
+	close_midx(tip);
 
-	return 0;
+	return ret;
 }
 
 static int read_midx_checksum(const char *object_dir)