contracts: fix sub overflow in pillage #853

Job	Run time
setup-environment	8s
test-scarb-fmt	11s
test-various (combat_test)	47m 47s
test-various (owner_test)	5m 16s
test-various (position_test)	1m 36s
test-various (resources_test)	27m 22s
test-various (config_test)	1m 37s
test-various (hyperstructure_test)	41m 29s
test-various (bank_test)	24m 14s
test-various (map_test)	17m 4s
test-various (resources_test)	27m 47s
test-various (trade_test)	37m 40s
test-various (realm_test)	22m 32s
test-various (guild_test)	39m 10s
test-various (transport_test)	15m 29s
test-various (test_ownership_systems)	5m 29s
	5h 14m 51s

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