JavaScript: Fix a bug in ServerSideUrlRedirect

[asger-semmle]

The server-side redirect query did not handle sinks in this case:

function f(res, nextUrl) {
  res.redirect(nextUrl);
}

because the logic in Sink.maybeNonLocal() required that a prefix of the sink must be an Expr. In this case, the prefix is found to be a ParameterNode whose asExpr() has no result.

I've refactored getAPrefix to return a DataFlow::Node and only coerce to Expr when doing getStringValue().

[xiemaisi]

LGTM