contracts: fix sub overflow in pillage #865

Job	Run time
setup-environment	7s
test-scarb-fmt	10s
test-various (combat_test)	38m 38s
test-various (owner_test)	5m 47s
test-various (position_test)	1m 47s
test-various (resources_test)	30m 4s
test-various (config_test)	1m 41s
test-various (hyperstructure_test)	37m 22s
test-various (bank_test)	28m 34s
test-various (map_test)	16m 48s
test-various (resources_test)	28m 39s
test-various (trade_test)	30m 9s
test-various (realm_test)	24m 52s
test-various (guild_test)	38m 29s
test-various (transport_test)	17m 26s
test-various (test_ownership_systems)	6m 3s
	5h 6m 36s

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