SIPGO library has response DoS vulnerability via nil pointer dereference

SIPGO is a library for writing SIP services in the GO language. Starting in version 0.3.0 and prior to version 1.0.0-alpha-1, a nil pointer dereference vulnerability is in the SIPGO library's `NewResponseFromRequest` function that affects all normal SIP operations. The vulnerability allows remote attackers to crash any SIP application by sending a single malformed SIP request without a To header. The vulnerability occurs when SIP message parsing succeeds for a request missing the To header, but the response creation code assumes the To header exists without proper nil checks. This affects routine operations like call setup, authentication, and message handling - not just error cases. This vulnerability affects all SIP applications using the sipgo library, not just specific configurations or edge cases, as long as they make use of the `NewResponseFromRequest` function. Version 1.0.0-alpha-1 contains a patch for the issue.