patch 8.0.0648: possible use of NULL pointer

Problem:    Possible use of NULL pointer if buflist_new() returns NULL.
            (Coverity)
Solution:   Check for NULL pointer in set_bufref().

src/buffer.c

@@ -372,7 +372,7 @@ open_buffer(
 set_bufref(bufref_T *bufref, buf_T *buf)
 {
     bufref->br_buf = buf;
-    bufref->br_fnum = buf->b_fnum;
+    bufref->br_fnum = buf == NULL ? 0 : buf->b_fnum;
     bufref->br_buf_free_count = buf_free_count;
 }
 

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    648,
 /**/
     647,
 /**/