patch 8.1.1207: some compilers give warning messages

Problem:    Some compilers give warning messages.
Solution:   Initialize variables, change printf() argument. (Christian
            Brabandt, closes #4305)
diff --git a/src/undo.c b/src/undo.c
index 978549a..1d26144 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -2271,7 +2271,7 @@
     u_header_T	    *uhp = NULL;
     u_header_T	    *last;
     int		    mark;
-    int		    nomark;
+    int		    nomark = 0;  // shut up compiler
     int		    round;
     int		    dosec = sec;
     int		    dofile = file;