fix-unit-occupancy: Handle invalid coordinates instead of crashing

It appears that positions of caged units brought by traders can be
invalid, in particular.

NEWS.rst

@@ -36,7 +36,7 @@ Fixes
 -----
 - Fixed a rare crash that could result from running `keybinding` in onLoadWorld.init
 - Script help that doesn't start with a space is now recognized correctly
-- `confirm`: Fixed issues with haul-delete, route-delete, and squad-disband confirmations intercepting keys too agressively
+- `confirm`: Fixed issues with haul-delete, route-delete, and squad-disband confirmations intercepting keys too aggressively
 - `fix-unit-occupancy`: Significantly optimized - up to 2,000 times faster in large fortresses
 - `gui/create-item`: Allow exiting quantity prompt
 - `modtools/create-unit`: Fixed a possible issue in reclaim fortress mode

plugins/fix-unit-occupancy.cpp

@@ -80,11 +80,16 @@ struct uo_buf {
     }
     inline uint8_t get (uint32_t x, uint32_t y, uint32_t z)
     {
-        return buf[offset(x, y, z)];
+        size_t off = offset(x, y, z);
+        if (off < size)
+            return buf[off];
+        return 0;
     }
     inline void set (uint32_t x, uint32_t y, uint32_t z, uint8_t val)
     {
-        buf[offset(x, y, z)] = val;
+        size_t off = offset(x, y, z);
+        if (off < size)
+            buf[off] = val;
     }
     inline void get_coords (size_t off, uint32_t &x, uint32_t &y, uint32_t &z)
     {