THRIFT-5725: Fix the comparison check for bio_get_fd

[psarkar24]

The routine bio_get_fd checks for the validity of a file descriptor by comparing the fd to be less
than or equal to zero. The issue is that fd of 0 is a valid file descriptor. This PR changes the check
to be less than zero.

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[Jens-G]

+1 good catch

[psarkar24]

@Jens-G If the patch looks good to you, can you please merge the changes? Thanks!