indexer: catch OOM when adding expected OIDs

When adding OIDs to the indexer's map of yet-to-be-seen OIDs to verify
that packfiles are complete, we do so by first allocating a new OID and
then calling `git_oidmap_set` on it. There was no check for memory
allocation errors in place, though, leading to possible segfaults due to
trying to copy data to a `NULL` pointer.

Verify the result of `git__malloc` with `GIT_ERROR_CHECK_ALLOC` to fix
the issue.

src/indexer.c

@@ -326,6 +326,7 @@ static int add_expected_oid(git_indexer *idx, const git_oid *oid)
 	    !git_oidmap_exists(idx->pack->idx_cache, oid) &&
 	    !git_oidmap_exists(idx->expected_oids, oid)) {
 		    git_oid *dup = git__malloc(sizeof(*oid));
+		    GIT_ERROR_CHECK_ALLOC(dup);
 		    git_oid_cpy(dup, oid);
 		    return git_oidmap_set(idx->expected_oids, dup, dup);
 	}