patch 8.1.1230: a lot of code is shared between vim.exe and gvim.exe

Problem:    A lot of code is shared between vim.exe and gvim.exe.
Solution:   Optionally put the shared code in vim.dll. (Ken Takata,
            closes #4287)

src/memline.c

@@ -4798,7 +4798,7 @@ findswapname(
 			// the messages displayed in the Vim window when
 			// loading a session from the .gvimrc file.
 			if (gui.starting && !gui.in_use)
-			    gui_start();
+			    gui_start(NULL);
 #endif
 			// Show info about the existing swap file.
 			attention_message(buf, fname);