heretic: replace shifted negative value with negative of shifted value

This fixes a "Shifting a negative value is undefined behaviour"
cppcheck error.

src/heretic/r_things.c

@@ -1006,7 +1006,7 @@ void R_DrawMasked(void)
 //
 // Added for the sideviewing with an external device
     if (viewangleoffset <= 1024 << ANGLETOFINESHIFT || viewangleoffset >=
-        -1024 << ANGLETOFINESHIFT)
+        -(1024 << ANGLETOFINESHIFT))
     {                           // don't draw on side views
         R_DrawPlayerSprites();
     }