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>

Author: SiddharthShrimali

builtin/submodule--helper.c

@@ -1059,7 +1059,7 @@ static void submodule_summary_callback(struct diff_queue_struct *q,
 
 		if (!S_ISGITLINK(p->one->mode) && !S_ISGITLINK(p->two->mode))
 			continue;
-		temp = (struct module_cb*)malloc(sizeof(struct module_cb));
+		temp = xmalloc(sizeof(*temp));
 		temp->mod_src = p->one->mode;
 		temp->mod_dst = p->two->mode;
 		temp->oid_src = p->one->oid;