Cheroot responds 500 when first header field line is a continuation

[kenballus]

❓ I'm submitting a ...

• 🐞 bug report
• 🐣 feature request
• ❓ question about the decisions made in the repository

🐞 Describe the bug. What is the current behavior?
When Cheroot receives a request in which the first field-line begins with SP or HTAB, it responds 500 because of an UnboundLocalError.

❓ What is the motivation / use case for changing the behavior?
Malformed requests should get 400 responses, and should not trigger unhandled exceptions.