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)

Author: brammool

src/testdir/test_vim9_expr.vim

@@ -2545,6 +2545,12 @@ def Test_expr7_namespace()
   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()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2549,
 /**/
     2548,
 /**/

src/vim9execute.c

@@ -1771,6 +1771,7 @@ call_def_function(
 		    tv->v_type = VAR_DICT;
 		    tv->v_lock = 0;
 		    tv->vval.v_dict = d;
+		    ++d->dv_refcount;
 		    ++ectx.ec_stack.ga_len;
 		}
 		break;