IMPALA-2954: Don't refer to potentially NULL runtime state
IMPALA-2954: Don't refer to potentially NULL runtime state

PlanFragmentExecutor::Cancel() tries to log the fragment instance ID
from the runtime state at VLOG_QUERY, but runtime state may be NULL if
Prepare() has not been called.

Change-Id: I7a21a0ef8721442e39b2873c5c2d8e91b442afb1
Reviewed-on: http://gerrit.cloudera.org:8080/2069
Reviewed-by: Tim Armstrong <2f8e34250f03a653da3dc27f7ec203eafdde80fc@cloudera.com>
Reviewed-by: Marcel Kornacker <44592b4eea36663c86b994bb0ea99d15309c1c7d@cloudera.com>
Tested-by: Internal Jenkins

kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala