kernel: add assert in _user_read_dir

* This will help catch bugs such as in #15607.

Change-Id: I25b28932f9db4e2abe8499dd829c910bb565086b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2082
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/system/kernel/fs/fd.cpp

@@ -984,6 +984,8 @@ _user_read_dir(int fd, struct dirent* userBuffer, size_t bufferSize,
 	if (status != B_OK)
 		return status;
 
+	ASSERT(count <= maxCount);
+
 	// copy the buffer back -- determine the total buffer size first
 	size_t sizeToCopy = 0;
 	BytePointer<struct dirent> entry = buffer;
@@ -993,6 +995,8 @@ _user_read_dir(int fd, struct dirent* userBuffer, size_t bufferSize,
 		entry += length;
 	}
 
+	ASSERT(sizeToCopy <= bufferSize);
+
 	if (user_memcpy(userBuffer, buffer, sizeToCopy) != B_OK)
 		return B_BAD_ADDRESS;