Bug 1873131 - Add some assertions to UniqueFileHandle. r=nika,tsmith

Differential Revision: https://phabricator.services.mozilla.com/D197748

mfbt/UniquePtrExtensions.h

@@ -11,6 +11,7 @@
 
 #include <type_traits>
 
+#include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/DebugOnly.h"
 #include "mozilla/fallible.h"
@@ -103,7 +104,11 @@ typedef int FileHandleType;
 #endif
 
 struct FileHandleHelper {
-  MOZ_IMPLICIT FileHandleHelper(FileHandleType aHandle) : mHandle(aHandle) {}
+  MOZ_IMPLICIT FileHandleHelper(FileHandleType aHandle) : mHandle(aHandle) {
+#if defined(XP_UNIX) && (defined(DEBUG) || defined(FUZZING))
+    MOZ_RELEASE_ASSERT(aHandle == kInvalidHandle || aHandle > 2);
+#endif
+  }
 
   MOZ_IMPLICIT constexpr FileHandleHelper(std::nullptr_t)
       : mHandle(kInvalidHandle) {}