Fix ordering bug in MessageSet parsing

MessageSet encodes protobuf groups each containing an
extension ID and a payload.  According to standard protobuf
conventions, these two fields can occur in either order within
the group.  Add two tests that check decoding of each possible
order and fix the bug that this uncovers.

Author: tbkka

Sources/SwiftProtobuf/BinaryDecoder.swift

@@ -1213,7 +1213,7 @@ internal struct BinaryDecoder: Decoder {
                         extDecoder.fieldWireFormat = .lengthDelimited
                         try extDecoder.decodeExtensionField(values: &values,
                                                             messageType: messageType,
-                                                            fieldNumber: fieldNumber,
+                                                            fieldNumber: ext.fieldNumber,
                                                             messageExtension: ext)
                         wasDecoded = extDecoder.consumed
                       }

Tests/SwiftProtobufTests/Test_MessageSet.swift

@@ -144,6 +144,44 @@ final class Test_MessageSet: XCTestCase {
     "}\n"
   )
 
+  func testParseOrder1() throws {
+    let serialized = Data([11,
+        16, 176, 166, 94, // Extension ID
+        26, 2, 120, 123, // Payload message
+        12])
+
+    let msg: SwiftProtoTesting_WireFormat_TestMessageSet
+    do {
+      msg = try SwiftProtoTesting_WireFormat_TestMessageSet(
+        serializedBytes: serialized,
+        extensions: SwiftProtoTesting_UnittestMset_Extensions)
+    } catch let e {
+      XCTFail("Failed to parse: \(e)")
+      return
+    }
+    XCTAssertEqual(
+      msg.SwiftProtoTesting_TestMessageSetExtension1_messageSetExtension.i, 123)
+  }
+
+  func testParseOrder2() throws {
+    let serialized = Data([11,
+        26, 2, 120, 123, // Payload message
+        16, 176, 166, 94, // Extension ID
+        12])
+
+    let msg: SwiftProtoTesting_WireFormat_TestMessageSet
+    do {
+      msg = try SwiftProtoTesting_WireFormat_TestMessageSet(
+        serializedBytes: serialized,
+        extensions: SwiftProtoTesting_UnittestMset_Extensions)
+    } catch let e {
+      XCTFail("Failed to parse: \(e)")
+      return
+    }
+    XCTAssertEqual(
+      msg.SwiftProtoTesting_TestMessageSetExtension1_messageSetExtension.i, 123)
+  }
+
   // text_format_unittest.cc: TEST_F(TextFormatMessageSetTest, Serialize)
   func testTextFormat_Serialize() {
     let msg = SwiftProtoTesting_TestMessageSetContainer.with {