Fix seqno mismatch and use-after-free in shp_sweep()

Fix seqno mismatch and use-after-free in shp_sweep()

The code wrote the swept sector after calling shp_check_one_mines().
This failed to use up the mine that hit the minesweeper, and triggered
a seqno mismatch oops.

The code wrote the minesweeper after calling shp_check_one_mines().
This used freed memory when the minesweeper got sunk there.

Broken in 4.0.17.  Fix by moving both calls before
shp_check_one_mines().
(cherry picked from commit b0644e822cd10d93c1168f7b356068fabc02b2bf)