add SAFE_DIV macro and usages

include/global.h

@@ -73,6 +73,14 @@
 #define abs(x) (((x) < 0) ? -(x) : (x))
 #endif
 
+// Used in cases where division by 0 can occur in the retail version.
+// Avoids invalid opcodes on some emulators, and the otherwise UB.
+#ifdef UBFIX
+#define SAFE_DIV(a, b) ((b) ? (a) / (b) : 0)
+#else
+#define SAFE_DIV(a, b) ((a) / (b))
+#endif
+
 // There are many quirks in the source code which have overarching behavioral differences from
 // a number of other files. For example, diploma.c seems to declare rodata before each use while
 // other files declare out of order and must be at the beginning. There are also a number of