patch 8.2.0791: a second popup window with terminal causes trouble

Problem:    A second popup window with terminal causes trouble.
Solution:   Disallow opening a second terminal-popup window. (closes #6101,
            closes #6103) Avoid defaulting to an invalid line number.
diff --git a/src/version.c b/src/version.c
index aa03f90..92f5660 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    791,
+/**/
     790,
 /**/
     789,