[patch] Adding an assertion to report too long hash table name

Hi,

The max size for the shmem hash table name is SHMEM_INDEX_KEYSIZE - 1.
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 -1 chars as the hash table name.

I created some shmem hash tables with the same prefix which was longer than
SHMEM_INDEX_KEYSIZE - 1, and the size of those hash tables were the same,
then only one hash table was created. But I thought those hash tables were created
successfully.

I know this is a corner case, but it's difficult to figure it out when run into it. So I add
an assertion to prevent it.

Thanks,
Xiaoran