patch 8.1.0589: compilation error in gvimext.cpp

Problem:    Compilation error in gvimext.cpp.
Solution:   Return a value.  Also fix using uninitialized variable.
diff --git a/src/dosinst.c b/src/dosinst.c
index f73576f..9ecb0f4 100644
--- a/src/dosinst.c
+++ b/src/dosinst.c
@@ -2459,7 +2459,7 @@
 	}
 	else if (strcmp(argv[i], "-create-directories") == 0)
 	{
-	    int vimfiles_dir_choice;
+	    int vimfiles_dir_choice = (int)vimfiles_dir_none;
 
 	    init_directories_choice();
 	    if (argv[i + 1][0] != '-')