t/helper/test-read-midx.c: free MIDX within read_midx_file()

t/helper/test-read-midx.c: free MIDX within read_midx_file()

When calling `read_midx_file()` to show information about a MIDX or list
the objects contained within it we fail to call `close_midx()`, leaking
the memory allocated to store that MIDX.

Fix this by calling `close_midx()` before exiting the function. We can
drop the "early" return when `show_objects` is non-zero, since the next
instruction is also a return.

(We could just as easily put a `cleanup` label here as with previous
patches. But the only other time we terminate the function early is
when we fail to load a MIDX in the first place. `close_midx()` does
handle a NULL argument, but the extra complexity is likely not
warranted).

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