Tweaks for SSI out-of-shared memory behavior.

Tweaks for SSI out-of-shared memory behavior.

If we call hash_search() with HASH_ENTER, it will bail out rather than
return NULL, so it's redundant to check for NULL again in the caller.
Thus, in cases where we believe it's impossible for the hash table to run
out of slots anyway, we can simplify the code slightly.

On the flip side, in cases where it's theoretically possible to run out of
space, we don't want to rely on dynahash.c to throw an error; instead,
we pass HASH_ENTER_NULL and throw the error ourselves if a NULL comes
back, so that we can provide a more descriptive error message.

Kevin Grittner