Please review (S): fix placeholder table entry cleanup

[Karen Kinnear]

Fix to clean up placeholdertable entries in stackoverflow conditions.

webrev: http://cr.openjdk.java.net/~acorn/7199207/webrev

bug link: https://jbs.oracle.com/bugs/browse/JDK-7199207

Thanks to Stefan Karlsson for identifying the problem and
the initial fix.

Tests run:
Originally reported chain007 with ThreadStackSize=128 and
original options.

vm.quick.testlist
vm.parallel_class_loading.testlist
jck: non-interactive jck, api, lang
jtreg: java.lang.instrument
RunThese full
ctw nightly.testlist

thanks,
Karen