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)

src/ex_eval.c

@@ -1173,7 +1173,8 @@ ex_else(exarg_T *eap)
     {
 	// 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);

src/testdir/test_vimscript.vim

@@ -192,6 +192,16 @@ func Test_if_while()
     call assert_equal('ab3j3b2c2b1f1h1km', g:Xpath)
 endfunc
 
+" Check double quote after skipped "elseif" does not give error E15
+func Test_skipped_elseif()
+  if "foo" ==? "foo"
+      let result = "first"
+  elseif "foo" ==? "foo"
+      let result = "second"
+  endif
+  call assert_equal('first', result)
+endfunc
+
 "-------------------------------------------------------------------------------
 " Test 4:   :return							    {{{1
 "-------------------------------------------------------------------------------

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    869,
 /**/
     868,
 /**/