viml/parser/expressions: Make sure that listed nodes may be present

With the new test leaving `assert(false);` for any of the cases makes tests 
crash.

src/nvim/viml/parser/expressions.c

@@ -3065,12 +3065,9 @@ viml_pexpr_parse_end:
           // to be caught later.
           break;
         }
+        case kExprNodeSubscript:
         case kExprNodeConcatOrSubscript:
         case kExprNodeComplexIdentifier:
-        case kExprNodeSubscript: {
-          // FIXME: Investigate whether above are OK to be present in the stack.
-          break;
-        }
         case kExprNodeAssignment:
         case kExprNodeMod:
         case kExprNodeDivision:

test/unit/viml/expressions/parser_tests.lua

@@ -4228,6 +4228,50 @@ return function(itp, _check_parsing, hl, fmtn)
       hl('ConcatOrSubscript', '.'),
       hl('Number', '1', 1),
     })
+
+    check_parsing('a[1][2][3[4', {
+      --           01234567890
+      --           0         1
+      ast = {
+        {
+          'Subscript:0:7:[',
+          children = {
+            {
+              'Subscript:0:4:[',
+              children = {
+                {
+                  'Subscript:0:1:[',
+                  children = {
+                    'PlainIdentifier(scope=0,ident=a):0:0:a',
+                    'Integer(val=1):0:2:1',
+                  },
+                },
+                'Integer(val=2):0:5:2',
+              },
+            },
+            {
+              'Subscript:0:9:[',
+              children = {
+                'Integer(val=3):0:8:3',
+                'Integer(val=4):0:10:4',
+              },
+            },
+          },
+        },
+      },
+    }, {
+      hl('IdentifierName', 'a'),
+      hl('SubscriptBracket', '['),
+      hl('Number', '1'),
+      hl('SubscriptBracket', ']'),
+      hl('SubscriptBracket', '['),
+      hl('Number', '2'),
+      hl('SubscriptBracket', ']'),
+      hl('SubscriptBracket', '['),
+      hl('Number', '3'),
+      hl('SubscriptBracket', '['),
+      hl('Number', '4'),
+    })
   end)
   itp('works with bracket subscripts', function()
     check_parsing(':', {