vppinfra: fix potential memory access error in _pool_init_fixed

vppinfra: fix potential memory access error in _pool_init_fixed

_pool_init_fixed uses mmap to initialize a fixed-size and preallocated
pool, whose size is the sum of vector_size and free_index_size with
alignment to the CLIB_CACHE_LINE_BYTES and page size. In this way
vector_size equals to pool_header_t + vec_header_t + elt_size * max_elts
so moving to the end of the pool space should be pool_header_t pointer +
vector_size, instead of vec_header_t pointer + vector_size.

Simple code to reproduce this error:

u64 *pool;
pool_init_fixed(pool, 2042);

Improve unit test to cover this case

Type: fix

Signed-off-by: Jieqiang Wang <jieqiang.wang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Change-Id: If088ef89b3dcb2d874ee837ae9da60983b14615c
Signed-off-by: Dave Barach <dave@barachs.net>