Add clusterfuzz cases that were found while a fix was already pending.

FuzzTesting/FailCases/clusterfuzz-testcase-minimized-FuzzJSON_debug-4506617283477504

@@ -0,0 +1 @@
+{"[fuzz.testing.singular_bytes_ext]":null

FuzzTesting/FailCases/clusterfuzz-testcase-minimized-FuzzJSON_release-5689942715006976

@@ -0,0 +1 @@
+{"[fuzz.testing.singular_enum_ext]":null

Tests/SwiftProtobufTests/Test_FuzzTests.swift

@@ -90,6 +90,8 @@ class Test_FuzzTests: XCTestCase {
       0x5c, 0x00, 0x2e, 0x20
     ])
 
+    // FailCases/clusterfuzz-testcase-minimized-FuzzJSON_debug-4506617283477504
+    // FailCases/clusterfuzz-testcase-minimized-FuzzJSON_release-5689942715006976
     assertJSONFails("{\"[fuzz.testing.singular_sint32_ext]\":null")
   }