patch 9.1.2111: Vim9: no error for elseif/else after else

Problem:  Vim9: no error for elseif/else after else
Solution: Report an error (Hirohito Higashi)

closes: #19263

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_script.vim

@@ -1995,6 +1995,25 @@ def Test_if_elseif_else_fails()
   END
   v9.CheckDefFailure(lines, 'E488:')
 
+
+  lines =<< trim END
+      if true
+      else
+      else
+      endif
+  END
+  v9.CheckSourceDefFailure(lines, 'E583:')
+
+  lines =<< trim END
+      var a = 3
+      if a == 2
+      else
+      elseif true
+      else
+      endif
+  END
+  v9.CheckSourceDefFailure(lines, 'E584:')
+
   lines =<< trim END
       var cond = true
       if cond

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2111,
 /**/
     2110,
 /**/

src/vim9cmds.c

@@ -596,6 +596,11 @@ compile_elseif(char_u *arg, cctx_T *cctx)
 	emsg(_(e_elseif_without_if));
 	return NULL;
     }
+    if (scope->se_u.se_if.is_seen_else)
+    {
+	emsg(_(e_elseif_after_else));
+	return NULL;
+    }
     unwind_locals(cctx, scope->se_local_count, TRUE);
     if (!cctx->ctx_had_return && !cctx->ctx_had_throw)
 	// the previous if block didn't end in a "return" or a "throw"
@@ -745,6 +750,11 @@ compile_else(char_u *arg, cctx_T *cctx)
 	emsg(_(e_else_without_if));
 	return NULL;
     }
+    if (scope->se_u.se_if.is_seen_else)
+    {
+	emsg(_(e_multiple_else));
+	return NULL;
+    }
     unwind_locals(cctx, scope->se_local_count, TRUE);
     if (!cctx->ctx_had_return && !cctx->ctx_had_throw)
 	// the previous if block didn't end in a "return" or a "throw"