libtailscale: fix bounds checks and nil pointer bugs in C API

[raggi]

• Fix bounds checking: check if copy() filled buffer (n >= len(out)) instead of incorrect len(out) < len(src)-1 (thank you @kitknox / out-of-bounds panic in buffer copy #45)
• Fix nil pointer dereference in 12 functions when getServer fails
• Fix syntax error in TsnetDial referencing undefined variable
• Simplify getServer to return *server instead of (*server, error) to avoid repetition of fixed bugs

Fixes #45

[Copilot]

Pull request overview

This PR fixes critical bugs in the C API including nil pointer dereferences, incorrect bounds checking logic, and syntax errors.

Changes:

• Fixed bounds checking logic to check n >= len(out) instead of incorrect len(out) < len(src)-1
• Fixed nil pointer dereferences in 12 functions by simplifying getServer to return *server and checking for nil
• Fixed syntax errors in TsnetDial and variable declaration in TsnetUp

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