cache: add a check for a failed allocation

Rather minimal change, but it's the kind of thing we should do.

src/cache.c

@@ -68,6 +68,7 @@ int git_cache_init(git_cache *cache)
 {
 	memset(cache, 0, sizeof(*cache));
 	cache->map = git_oidmap_alloc();
+	GITERR_CHECK_ALLOC(cache->map);
 	if (git_rwlock_init(&cache->lock)) {
 		giterr_set(GITERR_OS, "Failed to initialize cache rwlock");
 		return -1;