Prevent a crash when decoding a single value from the query container

Sources/Vapor/URLEncodedForm/URLEncodedFormDecoder.swift

@@ -124,7 +124,11 @@ private struct _Decoder: Decoder {
 
     /// See `Decoder`
     func singleValueContainer() throws -> SingleValueDecodingContainer {
-        return SingleValueContainer(data: self.data!, codingPath: codingPath)
+        guard let data = self.data else {
+            throw DecodingError.valueNotFound(Any.self, at: codingPath)
+        }
+
+        return SingleValueContainer(data: data, codingPath: codingPath)
     }
 
     struct SingleValueContainer: SingleValueDecodingContainer {

Tests/VaporTests/ApplicationTests.swift

@@ -78,11 +78,32 @@ final class ApplicationTests: XCTestCase {
         let request = Request(application: app, on: app.eventLoopGroup.next())
         request.headers.contentType = .json
         request.url.path = "/foo"
-        print(request.url.string)
         request.url.query = "hello=world"
-        print(request.url.string)
         try XCTAssertEqual(request.query.get(String.self, at: "hello"), "world")
     }
+
+    // https://github.com/vapor/vapor/pull/2163
+    func testWrappedSingleValueQueryDecoding() throws {
+        let app = Application()
+        defer { app.shutdown() }
+
+        let request = Request(application: app, on: app.eventLoopGroup.next())
+        request.headers.contentType = .json
+        request.url.path = "/foo"
+        request.url.query = ""
+
+        // Think of property wrappers, or MongoKitten's ObjectId
+        struct StringWrapper: Decodable {
+            let string: String
+
+            init(from decoder: Decoder) throws {
+                let container = try decoder.singleValueContainer()
+                string = try container.decode(String.self)
+            }
+        }
+
+        XCTAssertThrowsError(try request.query.get(StringWrapper.self, at: "hello"))
+    }
 
     func testQueryGet() throws {
         let app = Application()