patch 7.4.1137

Problem:    Illegal memory access when using :copen and :cclose.
Solution:   Avoid that curbuf is invalid. (suggestion by Justin M. Keyes)
            Add a test.

src/testdir/test_quickfix.vim

@@ -295,4 +295,10 @@ function Test_nomem()
 
 endfunc
 
+function Test_helpgrep()
+  helpgrep quickfix
+  copen
+  " This wipes out the buffer, make sure that doesn't cause trouble.
+  cclose
+endfunc
 

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1137,
 /**/
     1136,
 /**/

src/window.c

@@ -2445,6 +2445,10 @@ win_close(win, free_buf)
 	if (win_valid(win))
 	    win->w_closing = FALSE;
 #endif
+	/* Make sure curbuf is valid. It can become invalid if 'bufhidden' is
+	 * "wipe". */
+	if (!buf_valid(curbuf))
+	    curbuf = firstbuf;
     }
 
     if (only_one_window() && win_valid(win) && win->w_buffer == NULL