Correct introduced bug from 99fae02ed30f9bbf1055b27b493448fab772faa4

in GribRecord.cpp hasValue is used when reading the grib file, even
if the value is not defined.  It is also used in other places,
where it needs to test that the value is also defined.
Merge pull request #167 from seandepagnier/master

Correct introduced bug from 99fae02ed30f9bbf1055b27b493448fab772faa4