Parser::Declare follow-up to r38979.

BUG=

Review-Url: https://codereview.chromium.org/2292223004
Cr-Commit-Position: refs/heads/master@{#39053}

src/parsing/parser.cc

@@ -1677,6 +1677,7 @@ Variable* Parser::Declare(Declaration* declaration,
     } else {
       ReportMessage(MessageTemplate::kParamDupe);
     }
+    return nullptr;
   }
   if (sloppy_mode_block_scope_function_redefinition) {
     ++use_counts_[v8::Isolate::kSloppyModeBlockScopedFunctionRedefinition];