Cast the original rcode to (dns_ttl_t) when setting extended rcode

Cast the original rcode to (dns_ttl_t) when setting extended rcode

Shifting (signed) integer left could trigger undefined behaviour when
the shifted value would overflow into the sign bit (e.g. 2048).

The issue was found when using AFL++ and UBSAN:

    message.c:2274:33: runtime error: left shift of 2048 by 20 places cannot be represented in type 'int'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior message.c:2274:33 in

(cherry picked from commit a347641782dfb47aa45e6e8ffc9e0c6db4c07deb)