[Bugfix][BCRSMatrix] Use number of rows from source matrix as iteration bound in copy constructor

After the recent changes to BCRSMatrix, row and column sizes are now only
set by allocate() because several places assume a correlation between
the set number and rows and the allocation state. Thus the copy
constructor and the assignment operator don't directly copy those values
over, but initialize them to 0. The correct values are then set by
allocate. The row-wise build mode is a special case that requires
iterating over the source matrix rows before allocating memory.
Unfortunately, the copy constructor tried to use the (0-initialized) row
count of itself as iteration bound instead of the source matrix's one.

Fixed by doing the same thing as the assignment operator: Just use the
number from the source matrix.

Fixes FS #1394.

dune/istl/bcrsmatrix.hh

@@ -768,7 +768,7 @@ namespace Dune {
       if (_nnz<=0)
       {
         _nnz = 0;
-        for (size_type i=0; i<n; i++)
+        for (size_type i=0; i<Mat.n; i++)
           _nnz += Mat.r[i].getsize();
       }