midx.c: replace `xcalloc()` with `CALLOC_ARRAY()`

midx.c: replace `xcalloc()` with `CALLOC_ARRAY()`

Replace a direct invocation of Git's `xcalloc()` wrapper with the
`CALLOC_ARRAY()` macro instead.

The latter is preferred since it is more conventional in Git's codebase,
but also because it automatically picks the correct value for the record
size.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>