Reject unterminated CDR strings during deserialization

[Daniiiil1]

Summary

• validate the mandatory trailing null byte before accepting a non-empty CDR string
• cover both Cdr and FastCdr, for std::string and C-string deserialization
• restore the deserializer state and throw BadParamException for malformed input
• correct an existing malformed XCDRv1 map fixture exposed by strict validation

Testing

• 2,129/2,129 functional tests passed with ctest --test-dir build-verify -E "^cmake\.force_standard\." --output-on-failure
• added four regression tests covering both deserializers and both string APIs

Fixes #331