patch 8.1.0551: expression evaluation may repeat an error message

Problem:    Expression evaluation may repeat an error message. (Jason
            Franklin)
Solution:   Check for the value of did_emsg when giving an error
            for the :execute command.
diff --git a/src/version.c b/src/version.c
index d8d66f0..29f3ec5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    551,
+/**/
     550,
 /**/
     549,