RFR: CSR: VM_HeapDumper hits assert with bad dump_len

[Denghui Dong]

Hi,

Please review this draft of the CSR for jdk8u: VM_HeapDumper hits assert with 
bad dump_len

CSR: https://bugs.openjdk.java.net/browse/JDK-8235300

Thanks,
Denghui Dong