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hexen: Validate ACS instructions.

We do an array lookup based on the instruction we read, so it is
important that the instruction is within the bounds of the array. In
particular the instruction may an extension supported by an advanced
source port like ZDoom, that we don't support.

--- a/src/hexen/p_acs.c

+++ b/src/hexen/p_acs.c

@@ -717,6 +717,11 @@

     do

     {

         cmd = ReadCodeInt();

+        ACSAssert(cmd >= 0, "negative ACS instruction %d", cmd);

+        ACSAssert(cmd < arrlen(PCodeCmds),

+                  "invalid ACS instruction %d (maybe this WAD is designed "

+                  "for an advanced source port and is not vanilla "

+                  "compatible)", cmd);

         action = PCodeCmds[cmd]();

     } while (action == SCRIPT_CONTINUE);