Never crash-and-restart for bgworkers without shared memory access.

The motivation for a crash and restart cycle when a backend dies is
that it might have corrupted shared memory on the way down; and we
can't recover reliably except by reinitializing everything.  But that
doesn't apply to processes that don't touch shared memory.  Currently,
there's nothing to prevent a background worker that doesn't request
shared memory access from touching shared memory anyway, but that's a
separate bug.

Previous to this commit, the coding in postmaster.c was inconsistent:
an exit status other than 0 or 1 didn't provoke a crash-and-restart,
but failure to release the postmaster child slot did.  This change
makes those cases consistent.

src/backend/postmaster/postmaster.c

@@ -2864,17 +2864,17 @@ CleanupBackgroundWorker(int pid,
 				HandleChildCrash(pid, exitstatus, namebuf);
 				return true;
 			}
-		}
 
-		if (!ReleasePostmasterChildSlot(rw->rw_child_slot))
-		{
-			/*
-			 * Uh-oh, the child failed to clean itself up.  Treat as a crash
-			 * after all.
-			 */
-			rw->rw_crashed_at = GetCurrentTimestamp();
-			HandleChildCrash(pid, exitstatus, namebuf);
-			return true;
+			if (!ReleasePostmasterChildSlot(rw->rw_child_slot))
+			{
+				/*
+				 * Uh-oh, the child failed to clean itself up.  Treat as a
+				 * crash after all.
+				 */
+				rw->rw_crashed_at = GetCurrentTimestamp();
+				HandleChildCrash(pid, exitstatus, namebuf);
+				return true;
+			}
 		}
 
 		/* Get it out of the BackendList and clear out remaining data */