bugfix: rest: avoid segfault for invalid URI

`evhttp_uri_parse` can return a nullptr, for example when the URI
contains invalid characters (e.g. "%").
`GetQueryParameterFromUri` passes the output of `evhttp_uri_parse`
straight into `evhttp_uri_get_query`, which means that anyone calling
a REST endpoint in which query parameters are used (e.g. `rest_headers`)
can cause a segfault.

This bugfix is designed to be minimal and without additional behaviour change.
Follow-up work should be done to resolve this in a more general and robust way,
so not every endpoint has to handle it individually.

src/httpserver.cpp

@@ -673,6 +673,9 @@ std::optional<std::string> HTTPRequest::GetQueryParameter(const std::string& key
 std::optional<std::string> GetQueryParameterFromUri(const char* uri, const std::string& key)
 {
     evhttp_uri* uri_parsed{evhttp_uri_parse(uri)};
+    if (!uri_parsed) {
+        throw std::runtime_error("URI parsing failed, it likely contained RFC 3986 invalid characters");
+    }
     const char* query{evhttp_uri_get_query(uri_parsed)};
     std::optional<std::string> result;