Handle the case where we continue after locking a nonexistent file

The previous version of this would lead to big warnings and
impossibles. This doesn't seem like a useful case, but it's still
better to have better warning messages just in case it does
happen.

src/files.c

@@ -1675,6 +1675,7 @@ int retryct;
     if (lockfd == -1) {
         HUP raw_printf("Cannot open file %s. Is NetHack installed correctly?",
                        filename);
+        nesting--;
         return FALSE;
     }
     sflock.l_type = F_WRLCK;