patch 8.2.4643: Vim9: variable may be locked unintentionally

Problem:    Vim9: variable may be locked unintentionally.
Solution:   Clear "v_lock". (closes #10036)

src/testdir/test_vim9_builtin.vim

@@ -371,10 +371,17 @@ def Test_bufname()
   assert_fails('bufname([])', 'E1220:')
 enddef
 
+let s:bufnr_res = 0
+
 def Test_bufnr()
   var buf = bufnr()
   bufnr('%')->assert_equal(buf)
 
+  # check the lock is not taken over through the stack
+  const nr = 10
+  bufnr_res = bufnr()
+  bufnr_res = 12345
+
   buf = bufnr('Xdummy', true)
   buf->assert_notequal(-1)
   exe 'bwipe! ' .. buf

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4643,
 /**/
     4642,
 /**/

src/vim9execute.c

@@ -937,6 +937,7 @@ call_prepare(int argcount, typval_T *argvars, ectx_T *ectx)
     tv = STACK_TV_BOT(-1);
     tv->v_type = VAR_NUMBER;
     tv->vval.v_number = 0;
+    tv->v_lock = 0;
 
     return OK;
 }