patch 8.0.0607: after :bwipe + :new bufref might still be valid

Problem:    When creating a bufref, then using :bwipe and :new it might get
            the same memory and bufref_valid() returns true.
Solution:   Add br_fnum to check the buffer number didn't change.
diff --git a/src/if_py_both.h b/src/if_py_both.h
index 78c70e7..b6f232e 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -4311,7 +4311,7 @@
     static int
 SetBufferLine(buf_T *buf, PyInt n, PyObject *line, PyInt *len_change)
 {
-    bufref_T	save_curbuf = {NULL, 0};
+    bufref_T	save_curbuf = {NULL, 0, 0};
     win_T	*save_curwin = NULL;
     tabpage_T	*save_curtab = NULL;
 
@@ -4415,7 +4415,7 @@
 	PyObject *list,
 	PyInt *len_change)
 {
-    bufref_T	save_curbuf = {NULL, 0};
+    bufref_T	save_curbuf = {NULL, 0, 0};
     win_T	*save_curwin = NULL;
     tabpage_T	*save_curtab = NULL;
 
@@ -4616,7 +4616,7 @@
     static int
 InsertBufferLines(buf_T *buf, PyInt n, PyObject *lines, PyInt *len_change)
 {
-    bufref_T	save_curbuf = {NULL, 0};
+    bufref_T	save_curbuf = {NULL, 0, 0};
     win_T	*save_curwin = NULL;
     tabpage_T	*save_curtab = NULL;