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

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.

mentioned in merge request !98 (merged)

We should have this in 2.6, probably 2.6.1!

added 121 commits

• 603c9ce3...7cb26196 - 120 commits from branch core:master
• 2e18d84a - [gmres][bugfix] Fix double call of prec->post(x) when res is zero

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added 9 commits

• 2e18d84a...dc687ac2 - 8 commits from branch core:master
• 6f62d30a - [gmres][bugfix] Fix double call of prec->post(x) when res is zero

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merged

mentioned in commit dad6bc3c