Fix UBSan Warnings

- memcpy() shouldn't be called with NULL, even if length == 0
- In CMod_LoadBrushSides() j (from in->texinfo) could be -1,
  which of course is no valid array index

src/common/collision.c

@@ -1532,7 +1532,7 @@ CMod_LoadBrushSides(lump_t *l)
 			Com_Error(ERR_DROP, "Bad brushside texinfo");
 		}
 
-		out->surface = &map_surfaces[j];
+		out->surface = (j >= 0) ? &map_surfaces[j] : &nullsurface;
 	}
 }
 

src/common/szone.c

@@ -73,7 +73,8 @@ SZ_GetSpace(sizebuf_t *buf, int length)
 void
 SZ_Write(sizebuf_t *buf, void *data, int length)
 {
-	memcpy(SZ_GetSpace(buf, length), data, length);
+	if(length > 0)
+		memcpy(SZ_GetSpace(buf, length), data, length);
 }
 
 void