[RTL]

Improve RtlImageNtHeaderEx:
- Fix signed/unsigned mismatch when comparing NT header offset
- Simplify overflow checks
- Add missing overflow-into-systemspace check
CR-77 / CORE-8091 #resolve

svn path=/trunk/; revision=67140

reactos/boot/freeldr/freeldr/lib/rtl/libsupp.c

@@ -20,6 +20,8 @@
 
 #include <freeldr.h>
 
+PVOID MmHighestUserAddress = (PVOID)MI_HIGHEST_USER_ADDRESS;
+
 #if DBG
 VOID FASTCALL
 CHECK_PAGED_CODE_RTL(char *file, int line)