vine: count child_count properly in the transfer server process

[JinZhou5042]

Proposed Changes

I found this problamatic while investigating #4076

In the worker's transfer server process, the time when increasing the child_count is before we even check if the lnk is valid or if the fork was successful. This leads to inaccurate counting, especially when the connection is NULL (timeout) or the fork fails. The result is that the worker could block indefinitely in the waitpid call, waiting for a child process that doesn't exist.

The solution is to only increment the child_count after successfully forking a child process, and handle fork failures separately to ensure the count remains accurate, preventing unnecessary blocking.

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