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

src/backend/storage/ipc/dsm_registry.c

@@ -177,19 +177,22 @@ GetNamedDSMSegment(const char *name, size_t size,
 				(errmsg("requested DSM segment size does not match size of "
 						"existing segment")));
 	}
-	else if (!dsm_find_mapping(entry->handle))
+	else
 	{
-		/* Attach to existing segment. */
-		dsm_segment *seg = dsm_attach(entry->handle);
+		dsm_segment *seg = dsm_find_mapping(entry->handle);
+
+		/* If the existing segment is not already attached, attach it now. */
+		if (seg == NULL)
+		{
+			seg = dsm_attach(entry->handle);
+			if (seg == NULL)
+				elog(ERROR, "could not map dynamic shared memory segment");
+
+			dsm_pin_mapping(seg);
+		}
 
-		dsm_pin_mapping(seg);
 		ret = dsm_segment_address(seg);
 	}
-	else
-	{
-		/* Return address of an already-attached segment. */
-		ret = dsm_segment_address(dsm_find_mapping(entry->handle));
-	}
 
 	dshash_release_lock(dsm_registry_table, entry);
 	MemoryContextSwitchTo(oldcontext);