patch 8.0.0542: getpos() can return a negative line number

Problem:    getpos() can return a negative line number. (haya14busa)
Solution:   Handle a zero topline and botline. (closes #1613)
diff --git a/src/version.c b/src/version.c
index eef36b4..d22f165 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    542,
+/**/
     541,
 /**/
     540,