Fix possible NULL pointer dereference in GetNamedDSMSegment().

Fix possible NULL pointer dereference in GetNamedDSMSegment().

GetNamedDSMSegment() doesn't check whether dsm_attach() returns
NULL, which creates the possibility of a NULL pointer dereference
soon after.  To fix, emit an ERROR if dsm_attach() returns NULL.
This shouldn't happen, but it would be nice to avoid a segfault if
it does.  In passing, tidy up the surrounding code.

Reported-by: Tom Lane
Reviewed-by: Michael Paquier, Bharath Rupireddy
Discussion: https://postgr.es/m/3348869.1705854106%40sss.pgh.pa.us