vim-patch:8.2.3486: illegal memory access with invalid sequence of commands

Problem:    Illegal memory access with invalid sequence of commands.
Solution:   Do not call leave_block() when not in a try block. (closes vim/vim#8966)
            Reset did_emsg so that exception is shown as an error.
cce81e9673

Vim9script is N/A, which includes leave_block.

src/nvim/ex_eval.c

@@ -1083,7 +1083,7 @@ void ex_endwhile(exarg_T *eap)
   if (cstack->cs_looplevel <= 0 || cstack->cs_idx < 0) {
     eap->errmsg = err;
   } else {
-    fl =  cstack->cs_flags[cstack->cs_idx];
+    fl = cstack->cs_flags[cstack->cs_idx];
     if (!(fl & csf)) {
       // If we are in a ":while" or ":for" but used the wrong endloop
       // command, do not rewind to the next enclosing ":for"/":while".
@@ -1575,6 +1575,7 @@ void ex_endtry(exarg_T *eap)
 
     if (!(cstack->cs_flags[cstack->cs_idx] & CSF_TRY)) {
       eap->errmsg = get_end_emsg(cstack);
+
       // Find the matching ":try" and report what's missing.
       idx = cstack->cs_idx;
       do {
@@ -1594,6 +1595,9 @@ void ex_endtry(exarg_T *eap)
       if (current_exception) {
         discard_current_exception();
       }
+
+      // report eap->errmsg, also when there already was an error
+      did_emsg = false;
     } else {
       idx = cstack->cs_idx;
 
@@ -1664,8 +1668,10 @@ void ex_endtry(exarg_T *eap)
      */
     (void)cleanup_conditionals(cstack, CSF_TRY | CSF_SILENT, TRUE);
 
-    --cstack->cs_idx;
+    if (cstack->cs_idx >= 0 && (cstack->cs_flags[cstack->cs_idx] & CSF_TRY)) {
-    --cstack->cs_trylevel;
+      cstack->cs_idx--;
+    }
+    cstack->cs_trylevel--;
 
     if (!skip) {
       report_resume_pending(pending,

src/nvim/testdir/test_trycatch.vim

@@ -2067,5 +2067,26 @@ func Test_error_in_catch_and_finally()
   call delete('XtestCatchAndFinally')
 endfunc
 
+" This was causing an illegal memory access
+func Test_leave_block_in_endtry_not_called()
+  let lines =<< trim END
+      " vim9script
+      " try #
+      try "
+      for x in []
+      if
+      endwhile
+      if
+      endtry
+  END
+  call writefile(lines, 'XtestEndtry')
+  try
+    source XtestEndtry
+  catch /E171:/
+  endtry
+
+  call delete('XtestEndtry')
+endfunc
+
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