Fix use of uninitialized value issue from fuzzing

[shukitchan]

This pull request refactors the FNV-1a hash implementation to simplify initialization and improve code clarity. The main changes involve moving the FNV initial constants into the class definitions as constexpr static members, initializing member variables directly, and simplifying constructors.

Hash algorithm improvements:

• Moved FNV initial constants (FNV_INIT_32, FNV_INIT_64) into the respective classes (ATSHash32FNV1a, ATSHash64FNV1a) as static constexpr members and used them to initialize hval directly in the class definition (include/tscore/HashFNV.h). [1] [2]
• Updated the clear() methods to use the new fnv_init static member instead of external constants (src/tscore/HashFNV.cc). [1] [2]
• Simplified constructors for ATSHash32FNV1a and ATSHash64FNV1a by using default constructors instead of explicitly calling clear() (src/tscore/HashFNV.cc).

Code style and safety:

• Zero-initialized the type_buf buffer in Http3FrameFactory::create for improved safety (src/proxy/http3/Http3Frame.cc).

This fixes the reported fuzzing issues
https://oss-fuzz.com/testcase-detail/4669620266270720
https://oss-fuzz.com/testcase-detail/4793610426449920

[ezelkow1]

[approve ci freebsd clang-analyzer autest]