IMPALA-4542: Fix use-after-free in some BE tests
IMPALA-4542: Fix use-after-free in some BE tests

Change RuntimeState::ReleaseResources() to use cached ExecEnv pointer,
rather than singleton.

Change-Id: I5302d665756183289edf227588da1c094e197584
Testing: Ran affected BE tests under ASAN.
Reviewed-on: http://gerrit.cloudera.org:8080/5243
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Tested-by: Internal Jenkins

cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala