fix(inccommand): never preview if parsing command failed

`errormsg` is not always set when parsing the command failed (e.g. when
the range contains invalid marks). Check the return value is better.

src/nvim/ex_getln.c

@@ -2375,15 +2375,16 @@ static void cmdpreview_close_win(void)
 ///    6. Revert all changes made by the preview callback.
 static void cmdpreview_show(CommandLineState *s)
 {
+  emsg_off++;  // Block error reporting as parsing the command may produce error messages
+
   // Parse the command line and return if it fails.
   exarg_T ea;
   CmdParseInfo cmdinfo;
   // Copy the command line so we can modify it.
   char *cmdline = xstrdup((char *)ccline.cmdbuff);
   char *errormsg = NULL;
 
-  parse_cmdline(cmdline, &ea, &cmdinfo, &errormsg);
-  if (errormsg != NULL) {
+  if (!parse_cmdline(cmdline, &ea, &cmdinfo, &errormsg)) {
     goto end;
   }
 
@@ -2409,7 +2410,6 @@ static void cmdpreview_show(CommandLineState *s)
   cmdmod_T save_cmdmod = cmdmod;
 
   cmdpreview = true;
-  emsg_silent++;                 // Block error reporting as the command may be incomplete
   msg_silent++;                  // Block messages, namely ones that prompt
   block_autocmds();              // Block events
   garray_T save_view;
@@ -2496,7 +2496,6 @@ static void cmdpreview_show(CommandLineState *s)
   ga_clear(&save_view);
   unblock_autocmds();                  // Unblock events
   msg_silent--;                        // Unblock messages
-  emsg_silent--;                       // Unblock error reporting
 
   // Restore the window "view".
   curwin->w_cursor   = s->is_state.save_cursor;
@@ -2512,6 +2511,7 @@ static void cmdpreview_show(CommandLineState *s)
   }
 end:
   xfree(cmdline);
+  emsg_off--;  // Unblock error reporting
 }
 
 static int command_line_changed(CommandLineState *s)

test/functional/ui/inccommand_spec.lua

@@ -2947,6 +2947,32 @@ it(':substitute with inccommand, allows :redraw before first separator is typed
   ]])
 end)
 
+it(':substitute with inccommand, does nothing if range contains invalid marks', function()
+  local screen = Screen.new(30, 6)
+  clear()
+  command('set undolevels=-1')
+  common_setup(screen, 'split', 'test')
+  feed([[:'a,'bs]])
+  screen:expect([[
+    test                          |
+    {15:~                             }|
+    {15:~                             }|
+    {15:~                             }|
+    {15:~                             }|
+    :'a,'bs^                       |
+  ]])
+  feed('/')
+  screen:expect([[
+    test                          |
+    {15:~                             }|
+    {15:~                             }|
+    {15:~                             }|
+    {15:~                             }|
+    :'a,'bs/^                      |
+  ]])
+  eq('', eval('v:errmsg'))
+end)
+
 it(":substitute doesn't crash with inccommand, if undo is empty #12932", function()
   local screen = Screen.new(10,5)
   clear()