IMPALA-1068: COMPUTE STATS should place -1 in #NULLs

With IMPALA-1033 we disabled the counting of the number of NULLs in each column,
and that gave a 2x speed-up in the computation. But erroneously the value 0 was
being placed in the number of NULLs, instead of the correct -1 that indicates
'unknown'.

Change-Id: Ib882eb2a87e7e2469f606081cb2881461b441a45
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3377
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins