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/vim9execute.c b/src/vim9execute.c
index 232f047..e20c711 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1771,6 +1771,7 @@
 		    tv->v_type = VAR_DICT;
 		    tv->v_lock = 0;
 		    tv->vval.v_dict = d;
+		    ++d->dv_refcount;
 		    ++ectx.ec_stack.ga_len;
 		}
 		break;