Correct assumption that object constants must be null

It looks like code assumes object constants are null.

– Stack –
at [0x6b2e7d80] Lorg/jikesrvm/VM; _assertionFailure(Ljava/lang/String;Ljava/lang/String;)V at line 528
at [0x6b2e7d98] Lorg/jikesrvm/VM; _assert(ZLjava/lang/String;Ljava/lang/String;)V at line 511
at [0x6b2e7db4] Lorg/jikesrvm/VM; _assert(Z)V at line 489
at [0x6b2e7e08] Lorg/jikesrvm/osr/ia32/OSR_OptExecStateExtractor; getObjectFrom(II[BLorg/vmmagic/unboxed/Offset;Lorg/jikesrvm/osr/ia32/OSR_TempRegisters;)Ljava/lang/Object; at line 464
at [0x6b2e7ea4] Lorg/jikesrvm/osr/ia32/OSR_OptExecStateExtractor; getExecStateSequence(Lorg/jikesrvm/scheduler/VM_Thread;[BLorg/vmmagic/unboxed/Offset;Lorg/vmmagic/unboxed/Offset;ILorg/vmmagic/unboxed/Offset;Lorg/jikesrvm/osr/ia32/OSR_TempRegisters;Lorg/jikesrvm/osr/OSR_EncodedOSRMap;)Lorg/jikesrvm/osr/OSR_ExecutionState; at line 357
at [0x6b2e7f44] Lorg/jikesrvm/osr/ia32/OSR_OptExecStateExtractor; extractState(Lorg/jikesrvm/scheduler/VM_Thread;Lorg/vmmagic/unboxed/Offset;Lorg/vmmagic/unboxed/Offset;I)Lorg/jikesrvm/osr/OSR_ExecutionState; at line 124
at [0x6b2e7fbc] Lorg/jikesrvm/adaptive/OSR_OnStackReplacementPlan; execute()V at line 115
at [0x6b2e7fd8] Lorg/jikesrvm/adaptive/recompilation/VM_CompilationThread; run()V at line 55
at [0x6b2e8000] Lorg/jikesrvm/scheduler/VM_Thread; startoff()V at line 606

http://jikesrvm.anu.edu.au/cattrack/results/rvmx86lnx32.anu.edu.au/commit-x87.846/development/default/dacapo/chart/Output.txt