Avoid crash in varobj deletion

PR varobj/28131 points out a crash in the varobj deletion code.  It
took a while to reproduce this, but essentially what happens is that a
top-level varobj deletes its root object, then deletes the "dynamic"
object.  However, deletion of the dynamic object may cause
~py_varobj_iter to run, which in turn uses gdbpy_enter_varobj:

gdbpy_enter_varobj::gdbpy_enter_varobj (const struct varobj *var)
: gdbpy_enter (var->root->exp->gdbarch, var->root->exp->language_defn)
{
}

However, because var->root has already been destroyed, this is
invalid.

I've added a new test case.  This doesn't reliably crash, but the
problem can easily be seen under valgrind (and, I presume, with ASAN,
though I did not try this).

Tested on x86-64 Fedora 32.  I also propose putting this on the GDB 11
branch, with a suitable ChangeLog entry of course.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28131

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