patch 8.0.0605: the quickfix cached buffer may become invalid

Problem:    The buffer that quickfix caches for performance may become
            invalid. (Daniel Hahler)
Solution:   Reset qf_last_bufref in qf_init_ext(). (Daniel Hahler,
            closes #1728, closes #1676)

src/quickfix.c

@@ -157,6 +157,13 @@ static qf_info_T *ll_get_or_alloc_list(win_T *);
  */
 #define GET_LOC_LIST(wp) (IS_LL_WINDOW(wp) ? wp->w_llist_ref : wp->w_llist)
 
+/*
+ * Looking up a buffer can be slow if there are many.  Remember the last one
+ * to make this a lot faster if there are multiple matches in the same file.
+ */
+static char_u *qf_last_bufname = NULL;
+static bufref_T  qf_last_bufref = {NULL, 0};
+
 /*
  * Read the errorfile "efile" into memory, line by line, building the error
  * list. Set the error list's title to qf_title.
@@ -1151,6 +1158,10 @@ qf_init_ext(
     int		    retval = -1;	/* default: return error flag */
     int		    status;
 
+    /* Do not used the cached buffer, it may have been wiped out. */
+    vim_free(qf_last_bufname);
+    qf_last_bufname = NULL;
+
     vim_memset(&state, 0, sizeof(state));
     vim_memset(&fields, 0, sizeof(fields));
 #ifdef FEAT_MBYTE
@@ -1659,13 +1670,6 @@ copy_loclist(win_T *from, win_T *to)
     to->w_llist->qf_curlist = qi->qf_curlist;	/* current list */
 }
 
-/*
- * Looking up a buffer can be slow if there are many.  Remember the last one
- * to make this a lot faster if there are multiple matches in the same file.
- */
-static char_u *qf_last_bufname = NULL;
-static bufref_T  qf_last_bufref = {NULL, 0};
-
 /*
  * Get buffer number for file "directory/fname".
  * Also sets the b_has_qf_entry flag.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    605,
 /**/
     604,
 /**/