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

gdb/testsuite/gdb.python/py-mi-var-info-path-expression.exp

@@ -85,3 +85,15 @@ mi_gdb_test "-var-list-children c1.car.atom" \
 mi_gdb_test "-var-info-path-expression c1.car.atom.ival" \
   "\\^error,msg=\".*\"" \
   "-var-info-path-expression c1.car.atom.ival"
+
+
+# Regression test for a crasher that would occur when deleting a
+# varobj that held an iterator that hadn't yet been completed.
+# See PR varobj/28131.
+mi_gdb_test "-var-create c1_again * &c1" \
+   "\\^done.*" \
+   "-var-create c1_again * &c1"
+mi_gdb_test "-var-list-children c1_again 0 1" \
+  "\\^done,numchild=\"1\",children=.child=\{name=\"c1_again.car\".*" \
+  "-var-list-children c1_again"
+mi_delete_varobj c1_again "delete c1_again"

gdb/varobj.c

@@ -1844,10 +1844,12 @@ varobj::~varobj ()
     }
 #endif
 
+  /* This must be deleted before the root object, because Python-based
+     destructors need access to some components.  */
+  delete var->dynamic;
+
   if (is_root_p (var))
     delete var->root;
-
-  delete var->dynamic;
 }
 
 /* Return the type of the value that's stored in VAR,