prevent Astrogation / Sector map corruption (old)

[mnalis]

prevent corruption described in https://github.com/mnalis/ironseed_fpc/issues/46

[mnalis]

was fixed in https://github.com/nukebloodaxe/ironseed_fpc/pull/16

Due to pointer usage and nonexistant array bounds checking, the game will sometimes corrupt memory following var screen: screentype, which happens to be var systems: systemarray, leading to crashes later in game.

This pull request:

• provides canary buffer, so buffer overflows on screen should not corrupt systems array
• checks the canary and informs when it has been corrupted, alowing for quick detection of bug when it happens
• adds optional debug flag to Makefile to enforce assert()s and allow gdb(1) debugging
• updates asserts() checks