merge-file: fix memory leaks on error path

merge-file: fix memory leaks on error path

Fix a memory leak in "merge-file", we need to loop over the "mmfs"
array and free() what we've got so far when we error out. As a result
we can mark a test as passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>