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>

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