patch 8.2.4675: no error for missing expression after :elseif

Problem:    No error for missing expression after :elseif. (Ernie Rael)
Solution:   Check for missing expression. (closes #10068)

src/ex_eval.c

@@ -1146,7 +1146,12 @@ ex_else(exarg_T *eap)
 
     if (eap->cmdidx == CMD_elseif)
     {
-	result = eval_to_bool(eap->arg, &error, eap, skip);
+	// When skipping we ignore most errors, but a missing expression is
+	// wrong, perhaps it should have been "else".
+	if (skip && ends_excmd(*eap->arg))
+	    semsg(_(e_invalid_expression_str), eap->arg);
+	else
+	    result = eval_to_bool(eap->arg, &error, eap, skip);
 
 	// When throwing error exceptions, we want to throw always the first
 	// of several errors in a row.  This is what actually happens when

src/testdir/test_vim9_script.vim

@@ -1605,6 +1605,16 @@ def Test_if_elseif_else_fails()
       endif
   END
   v9.CheckDefFailure(lines, 'E488:')
+
+  lines =<< trim END
+      var cond = true
+      if cond
+        echo 'true'
+      elseif
+        echo 'false'
+      endif
+  END
+  v9.CheckDefAndScriptFailure(lines, ['E1143:', 'E15:'], 4)
 enddef
 
 let g:bool_true = v:true

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4675,
 /**/
     4674,
 /**/