RFR: 8252997: Null-proofing for linker_md.c

[Adam Farley8]

Hi All,

Small change proposed here to protect against null return values from 
jvmtiAllocation.

Requesting reviews and sponsor, please.

Bug: https://bugs.openjdk.java.net/browse/JDK-8252997

Webrev: http://cr.openjdk.java.net/~afarley/8252997/webrev/ 

Best Regards

Adam Farley 
IBM Runtimes

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