patch 8.2.1672: v_lock is used when it is not initialized

Problem:    v_lock is used when it is not initialized. (Yegappan Lakshmanan)
Solution:   Initialize the typval in eval1().
diff --git a/src/eval.c b/src/eval.c
index 3e972a2..18f3a5b 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2103,6 +2103,8 @@
     char_u  *p;
     int	    getnext;
 
+    CLEAR_POINTER(rettv);
+
     /*
      * Get the first variable.
      */