patch 9.0.0869: bogus error when string used after :elseif

Problem:    Bogus error when string used after :elseif.
Solution:   Do not consider a double quote the start of a comment.
            (closes #11534)
diff --git a/src/ex_eval.c b/src/ex_eval.c
index e1c2889..68dc6d7 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -1173,7 +1173,8 @@
     {
 	// When skipping we ignore most errors, but a missing expression is
 	// wrong, perhaps it should have been "else".
-	if (skip && ends_excmd(*eap->arg))
+	// A double quote here is the start of a string, not a comment.
+	if (skip && *eap->arg != '"' && ends_excmd(*eap->arg))
 	    semsg(_(e_invalid_expression_str), eap->arg);
 	else
 	    result = eval_to_bool(eap->arg, &error, eap, skip, FALSE);