[ubsan] Fix a faulty memory accessibility check

The dynamic type check needs to inspect vtables, but could crash if it
encounters a vtable pointer to inaccessible memory. In the first attempt
to fix the issue (r304437), we performed a memory accessibility check on
the wrong range of memory. This should *really* fix the problem.

Patch by Max Moroz!

Differential Revision: https://reviews.llvm.org/D34215

llvm-svn: 305489