fix(api): use a conditional stack for nvim_cmd

(cherry picked from commit c0b9bdc4a3)

src/nvim/ex_docmd.c

@@ -1694,6 +1694,9 @@ int execute_cmd(exarg_T *eap, CmdParseInfo *cmdinfo, bool preview)
     goto end;
   }
 
+  cstack_T cstack = { .cs_idx = -1 };
+  eap->cstack = &cstack;
+
   // Execute the command
   execute_cmd0(&retv, eap, &errormsg, preview);