implement newOrNil, changes to stdlib

- newOrNil works like new, but sets the variable to nil
  if the heap allocation failed
- change stdlib to use newOrNil in openfile and openvolumeid
- changes to programs that use openvolumeid

lib/corelib.s

@@ -612,6 +612,9 @@ WAIT1LOOP:
 ; length must be multiple of wordsize.
 ; if it is not, the last (partial) word is not cleared.
 _CLEARMEM:
+	OVER	; check for null pointer
+	CBRANCH.Z CLEARMEM_X
+
 	SHR
 	SHR	; calculate length in words