Add error check for invalid JSX attr initializer

Summary:
There was an invalid assert as a result of using `else if` instead of
`else`. Fix the case and ensure that one of the two branches execute.

Reviewed By: tmikov

Differential Revision: D30710922

fbshipit-source-id: de48ece5a6d87deb3df72ba01e1a71d38076c607

lib/Parser/JSParserImpl-jsx.cpp

@@ -334,9 +334,16 @@ Optional<ESTree::Node *> JSParserImpl::parseJSXAttribute() {
         tok_,
         new (context_) ESTree::StringLiteralNode(tok_->getStringLiteral()));
     advance(JSLexer::GrammarContext::AllowJSXIdentifier);
-  } else if (check(TokenKind::l_brace)) {
+  } else {
     // { AssignmentExpression }
     // ^
+    if (!need(
+            TokenKind::l_brace,
+            "in JSX attribute",
+            "location of attribute",
+            start))
+      return None;
+
     SMLoc valueStart = advance().Start;
 
     auto optAssign = parseAssignmentExpression();

test/Parser/jsx-error-attr.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// RUN: (! %hermes -parse-jsx -dump-ast -pretty-json %s 2>&1 ) | %FileCheck %s --match-full-lines
+
+<foo x=y></foo>
+// CHECK:  {{.*}}:10:8: error: '{' expected in JSX attribute
+// CHECK-NEXT: <foo x=y></foo>
+// CHECK-NEXT:      ~~^