IMPALA-774: Fix runtimestate setup when evaluating expr from FE.

We weren't initializing the udf mem pool causing UDFs to return strings to crash if used as part
of a constant expression.

Change-Id: Ic3a0e556aec8ce03a9e59f3ccf6980c682046b50
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1393
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins