Add an assert to the length of shared hashtable name

The max size for the shared memory hash table name is SHMEM_INDEX_KEYSIZE - 1
(shared hash table name is stored and indexed by ShmemIndex hash table,
the key size of it is SHMEM_INDEX_KEYSIZE), but when the caller uses a
longer hash table name, it doesn't report any error, instead it just
uses the first SHMEM_INDEX_KEYSIZE chars as the hash table name.

When some hash tables' names have the same prefix which is longer than
(SHMEM_INDEX_KEYSIZE - 1), issues will come: those hash tables actually
are created as the same hash table whose name is the prefix. So add the
assert to prevent it.