patch 8.1.0060: crash when autocommands delete the current buffer

Problem:    Crash when autocommands delete the current buffer. (Dominique
            Pelle)
Solution:   Check that autocommands don't change the buffer.
diff --git a/src/quickfix.c b/src/quickfix.c
index 1b281ce..031c6e7 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -6272,8 +6272,16 @@
 	    if (res >= 0)
 		qf_list_changed(qi, qi->qf_curlist);
 	    if (au_name != NULL)
+	    {
+		buf_T *curbuf_old = curbuf;
+
 		apply_autocmds(EVENT_QUICKFIXCMDPOST, au_name,
 						curbuf->b_fname, TRUE, curbuf);
+		if (curbuf != curbuf_old)
+		    // Autocommands changed buffer, don't jump now, "qi" may
+		    // be invalid.
+		    res = 0;
+	    }
 	    if (res > 0 && (eap->cmdidx == CMD_cbuffer ||
 						eap->cmdidx == CMD_lbuffer))
 		qf_jump(qi, 0, 0, eap->forceit);  /* display first error */