lib/bcheck: Fix code typo in __bound_delete_region()

We were calling get_page() with t2 index which is not correct, since
get_page() operate on t1 indices. The bug is here from day-1, from
60f781c4 (first version of bounds checker) and show as a crash in
__bound_delete_region() at program exit:

    $ ./tcc   -B. -DTCC_TARGET_I386 -DCONFIG_MULTIARCHDIR=\"i386-linux-gnu\" -b -run -DONE_SOURCE \
      ./tcc.c -B. -DTCC_TARGET_I386 -DCONFIG_MULTIARCHDIR=\"i386-linux-gnu\"    -run -DONE_SOURCE \
      ./tcc.c -B. -run tests/tcctest.c

    (lot's of correct output from tcctest)
    Runtime error: dereferencing invalid pointer
    at 0xa7c21cc4 __bound_delete_region()
    by (nil) ???
    Segmentation fault

The fix is simple - last page should be get through t1_end, like it is
done in __bound_new_region().

After this patch, tcc is being able to compile itself with -b, then
compile itself again and run tcctest with correct output. Tests follow.

lib/bcheck.c

@@ -613,7 +613,7 @@ int __bound_delete_region(void *p)
             }
         }
         /* last page */
-        page = get_page(t2_end);
+        page = get_page(t1_end);
         e2 = (BoundEntry *)((char *)page + t2_end);
         for(e=page;e<e2;e++) {
             e->start = 0;