Fix creation of initial value for computing the max (Fixes PR #805).

Bård spotet a bug after PR #805 was merged. Indead returning
-numeric_limits<type>::min() does not make sense for integral
values. This commit resorts to returning numeric_limits<type>::min().

Kudos to Bård for his attention.

opm/core/linalg/ParallelIstlInformation.hpp

@@ -481,7 +481,7 @@ private:
             // for integral types.
             if( std::is_integral<Result>::value )
             {
-                return -std::numeric_limits<Result>::min();
+                return std::numeric_limits<Result>::min();
             }
             else
             {