patch 8.1.0804: crash when setting v:errmsg to empty list

Problem:    Crash when setting v:errmsg to empty list. (Jaon Franklin)
Solution:   Separate getting value and assigning result.
diff --git a/src/version.c b/src/version.c
index dcbf1c2..8706d98 100644
--- a/src/version.c
+++ b/src/version.c
@@ -792,6 +792,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    804,
+/**/
     803,
 /**/
     802,