patch 8.0.1475: invalid memory access in read_redo()

Problem:    Invalid memory access in read_redo(). (gy741)
Solution:   Convert the replacement character back from a negative number to
            CR or NL. (hint by Dominique Pelle, closes #2616)
diff --git a/src/vim.h b/src/vim.h
index ad37235..bd8ac08 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2515,4 +2515,8 @@
 # endif
 #endif
 
+/* Replacement for nchar used by nv_replace(). */
+#define REPLACE_CR_NCHAR    -1
+#define REPLACE_NL_NCHAR    -2
+
 #endif /* VIM__H */