Fixed|DEH Reader: Crash parsing DEH patches referencing unknown defin…

…itions

DeHackEd patches are intended for a specific version of a game, which
we may not have default definitions for. In such a case, read into a
dummy definition so at least parsing *should* complete successfully.

Encountered this when regression testing Tango's MBF-compatible mod
"Mayan Mishap" (see: http://www.doomworld.com/vb/wads-mods/70845-mayan-mishap-final-release/)

(Although we don't presently support the MBF feature set, Doomsday
should (ideally) not crash when trying a mod that requires it).

doomsday/libdoomsday/include/doomsday/defs/dedarray.h

@@ -128,6 +128,16 @@ struct LIBDOOMSDAY_PUBLIC DEDArray
         return elements[index];
     }
 
+    PODType const &first() const
+    {
+        return at(0);
+    }
+
+    PODType const &last() const
+    {
+        return at(size() - 1);
+    }
+
     /**
      * Appends new, zeroed elements to the end of the array.
      *
@@ -199,9 +209,9 @@ struct LIBDOOMSDAY_PUBLIC DEDArray
 
     int indexOf(PODType const *element) const
     {
-        int index = element - elements;
-        DENG2_ASSERT(index >= 0 && index < size());
-        return index;
+        if(size() > 0 && element >= &first() && element <= &last())
+            return element - elements;
+        return -1;
     }
 
     void clear()