RFR(XXS): 8206254: Unable to complete emergency dump during safepoint

[Markus Gronlund]

Greetings,

Please review this small adjustment in order to allow writing an emergency dump from within a safepoint context.

Bug: https://bugs.openjdk.java.net/browse/JDK-8206254 
Webrev: http://cr.openjdk.java.net/~mgronlun/8206254/webrev00/

Thanks
Markus