patch 8.2.1704: Vim9: crash in for loop when autoload script has an error

Problem:    Vim9: crash in for loop when autoload script has an error.
Solution:   Reset suppress_errthrow.  Check for NULL list. (closes #6967)

src/testdir/test_vim9_script.vim

@@ -3353,6 +3353,45 @@ def Test_vim9_autoload()
   &rtp = save_rtp
 enddef
 
+" This was causing a crash because suppress_errthrow wasn't reset.
+def Test_vim9_autoload_error()
+  let lines =<< trim END
+      vim9script
+      def crash#func()
+          try
+              for x in List()
+              endfor
+          catch
+          endtry
+          g:ok = true
+      enddef
+      fu List()
+          invalid
+      endfu
+      try
+          invalid
+      catch /wontmatch/
+      endtry
+  END
+  call mkdir('Xruntime/autoload', 'p')
+  call writefile(lines, 'Xruntime/autoload/crash.vim')
+
+  # run in a separate Vim to avoid the side effects of assert_fails()
+  lines =<< trim END
+    exe 'set rtp^=' .. getcwd() .. '/Xruntime'
+    call crash#func()
+    call writefile(['ok'], 'Xdidit')
+    qall
+  END
+  writefile(lines, 'Xscript')
+  RunVim([], [], '-S Xscript')
+  assert_equal(['ok'], readfile('Xdidit'))
+
+  delete('Xdidit')
+  delete('Xscript')
+  delete('Xruntime', 'rf')
+enddef
+
 def Test_script_var_in_autocmd()
   # using a script variable from an autocommand, defined in a :def function in a
   # legacy Vim script, cannot check the variable type.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1704,
 /**/
     1703,
 /**/

src/vim9execute.c

@@ -761,6 +761,7 @@ call_def_function(
     sctx_T	save_current_sctx = current_sctx;
     int		breakcheck_count = 0;
     int		called_emsg_before = called_emsg;
+    int		save_suppress_errthrow = suppress_errthrow;
 
 // Get pointer to item in the stack.
 #define STACK_TV(idx) (((typval_T *)ectx.ec_stack.ga_data) + idx)
@@ -907,6 +908,9 @@ call_def_function(
     current_sctx = ufunc->uf_script_ctx;
     current_sctx.sc_version = SCRIPT_VERSION_VIM9;
 
+    // Do turn errors into exceptions.
+    suppress_errthrow = FALSE;
+
     // Decide where to start execution, handles optional arguments.
     init_instr_idx(ufunc, argc, &ectx);
 
@@ -1884,7 +1888,8 @@ call_def_function(
 		    // push the next item from the list
 		    if (GA_GROW(&ectx.ec_stack, 1) == FAIL)
 			goto failed;
-		    if (++idxtv->vval.v_number >= list->lv_len)
+		    ++idxtv->vval.v_number;
+		    if (list == NULL || idxtv->vval.v_number >= list->lv_len)
 			// past the end of the list, jump to "endfor"
 			ectx.ec_iidx = iptr->isn_arg.forloop.for_end;
 		    else if (list->lv_first == &range_list_item)
@@ -2713,6 +2718,9 @@ failed_early:
     vim_free(ectx.ec_stack.ga_data);
     vim_free(ectx.ec_trystack.ga_data);
 
+    // Not sure if this is necessary.
+    suppress_errthrow = save_suppress_errthrow;
+
     if (ret != OK && called_emsg == called_emsg_before)
 	semsg(_(e_unknown_error_while_executing_str),
 						   printable_func_name(ufunc));