patch 8.2.4840: heredoc expression evaluated even when skipping

Problem:    Heredoc expression evaluated even when skipping.
Solution:   Don't evaluate when "skip" is set. (closes #10306)
diff --git a/src/evalvars.c b/src/evalvars.c
index ffa7e93..21a0e5b 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -835,7 +835,7 @@
 	}
 	else
 	{
-	    if (evalstr)
+	    if (evalstr && !eap->skip)
 	    {
 		str = eval_all_expr_in_str(str);
 		if (str == NULL)