Suppress a couple of compiler warnings seen with older gcc versions.

To wit,
bgworker.c: In function `RegisterDynamicBackgroundWorker':
bgworker.c:761: warning: `generation' might be used uninitialized in this function
dsm_impl.c: In function `dsm_impl_op':
dsm_impl.c:197: warning: control reaches end of non-void function

Neither of these represent actual bugs, but we may as well tweak the code
so that more compilers can tell that.  This won't change the generated code
on compilers that do recognize that the cases are unreachable.

src/backend/postmaster/bgworker.c

@@ -758,7 +758,7 @@ RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
 {
 	int		slotno;
 	bool	success = false;
-	uint64	generation;
+	uint64	generation = 0;
 
 	/*
 	 * We can't register dynamic background workers from the postmaster.

src/backend/storage/ipc/dsm_impl.c

@@ -191,9 +191,11 @@ dsm_impl_op(dsm_op op, dsm_handle handle, uint64 request_size,
 			return dsm_impl_mmap(op, handle, request_size, impl_private,
 								 mapped_address, mapped_size, elevel);
 #endif
+		default:
+			elog(ERROR, "unexpected dynamic shared memory type: %d",
+				 dynamic_shared_memory_type);
+			return false;
 	}
-	elog(ERROR, "unexpected dynamic shared memory type: %d",
-		 dynamic_shared_memory_type);
 }
 
 /*