Ensure one can call DisposeEnvironment during event callbacks.

Previously calling DisposeEnvironment during an event callback would
cause a currently in-use iterator to be invalidated. This could cause
undefined behavior.

Bug: 37283252
Test: ./test/testrunner/testrunner.py --host --jvmti-stress -j40
Change-Id: I49c02b925f0da5d4f66965f0fca21bf8dc83132a
4 files changed