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.

test/functional/interface_rest.py

@@ -277,6 +277,10 @@ class RESTTest (BitcoinTestFramework):
         assert_equal(len(json_obj), 1)  # ensure that there is one header in the json response
         assert_equal(json_obj[0]['hash'], bb_hash)  # request/response hash should be the same
 
+        # Check invalid uri (% symbol at the end of the request)
+        resp = self.test_rest_request(f"/headers/{bb_hash}%", ret_type=RetType.OBJ, status=400)
+        assert_equal(resp.read().decode('utf-8').rstrip(), "URI parsing failed, it likely contained RFC 3986 invalid characters")
+
         # Compare with normal RPC block response
         rpc_block_json = self.nodes[0].getblock(bb_hash)
         for key in ['hash', 'confirmations', 'height', 'version', 'merkleroot', 'time', 'nonce', 'bits', 'difficulty', 'chainwork', 'previousblockhash']: