patch 8.2.2549: crash after using "g:" in a for loop

Problem:    Crash after using "g:" in a for loop.
Solution:   Increment the reference count. (closes #7892)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 3e69a0d..cda1ffd 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2545,6 +2545,12 @@
   assert_equal('some', get(t:, 'some_var', 'xxx'))
   assert_equal('xxx', get(t:, 'no_var', 'xxx'))
   unlet t:some_var
+
+  # check using g: in a for loop more than DO_NOT_FREE_CNT times
+  for i in range(100000)
+    if has_key(g:, 'does-not-exist')
+    endif
+  endfor
 enddef
 
 def Test_expr7_parens()