If the first argument to GeomFromWKB function is a geometry
field then the function just returns its value.
However in doing so it's not preserving first argument's 
null_value flag and this causes unexpected null value to
be returned to the calling function.
      
Fixed by updating the null_value of the GeomFromWKB function
in such cases (and all other cases that return a NULL e.g.
because of not enough memory for the return buffer).