Use standard offsetof facility. Dereferencing a null pointer results

in undefined behaviour.

code/qcommon/q_shared.h

@@ -160,6 +160,7 @@ typedef int intptr_t;
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <time.h>
 #include <ctype.h>

code/renderergl1/tr_bsp.c

@@ -330,7 +330,7 @@ static void ParseFace( dsurface_t *ds, drawVert_t *verts, msurface_t *surf, int
 	numIndexes = LittleLong( ds->numIndexes );
 
 	// create the srfSurfaceFace_t
-	sfaceSize = ( size_t ) &((srfSurfaceFace_t *)0)->points[numPoints];
+	sfaceSize = offsetof( srfSurfaceFace_t, points ) + sizeof( *cv->points ) * numPoints;
 	ofsIndexes = sfaceSize;
 	sfaceSize += sizeof( int ) * numIndexes;