[!172] [gmres][bugfix] Fix double call of prec->post(x) when res is zero

Merge branch 'feature/gmres-fix-prec-call-zeroresidual' into 'master'

ref:core/dune-istl The post-processing routine was called twice, once before
the outer while loop and once after when the residual was initially zero. This
lead to a segmentation fault in the AMG preconditioner. Now it only gets
called once.

See merge request [!172]

  [!172]: gitlab.dune-project.org/core/dune-istl/merge_requests/172

dune/istl/solvers.hh

@@ -1123,8 +1123,8 @@ namespace Dune {
           }
         }
 
-      if(Simd::allTrue(norm_0 < EPSILON)) {
-        _prec->post(x);
+      if(Simd::allTrue(norm_0 < EPSILON))
+      {
         res.converged = true;
         if(_verbose > 0) // final print
           print_result(res);