FIX: Prevent null-byte searches causing 500 error (#8226)

This fix ensures that searches that contain a null byte return a 400
error instead of causing a 500 error.

For some reason from rspec we will reach the raise statement inside
of the `rescue_from ArgumentError` block, but outside of rspec it will
not execute the raise statement and so a 500 is thrown instead of
reaching the `rescue_from Discourse::InvalidParameters` block inside of
the application controller.

This fix raises Discourse::InvalidParameters directly from the search
controller instead of relying on `PG::Connection.escape_string` to
raise the `ArgumentError`.

spec/requests/search_controller_spec.rb

@@ -197,6 +197,13 @@ describe SearchController do
       expect(response.status).to eq(400)
     end
 
+    it "returns a 400 error if you search for null bytes" do
+      term = "hello\0hello"
+
+      get "/search.json", params: { q: term }
+      expect(response.status).to eq(400)
+    end
+
     it "logs the search term" do
       SiteSetting.log_search_queries = true
       get "/search.json", params: { q: 'bantha' }