submodule--helper: replace malloc with xmalloc

submodule--helper: replace malloc with xmalloc

The submodule_summary_callback() function currently uses a raw malloc()
which could lead to a NULL pointer dereference.

Standardize this by replacing malloc() with xmalloc() for error handling.
To improve maintainability, use sizeof(*temp) instead of the struct name,
and drop the typecast of void pointer assignment.

Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>