CLEANUP: quic: replace ALREADY_CHECKED() with ASSUME_NONNULL() at a few places

CLEANUP: quic: replace ALREADY_CHECKED() with ASSUME_NONNULL() at a few places

There were 4 instances of ALREADY_CHECKED() used to tell the compiler that
the argument couldn't be NULL by design. Let's change them to the cleaner
ASSUME_NONNULL(). Functions like qc_snd_buf() were slightly reduced in
size (-24 bytes).

Apparently gcc-13 sees a potential case that others don't see, and it's
likely a bug since depending what is masked, it will completely change
the output warnings to the point of contradicting itself. After many
attempts, it appears that just checking that CMSG_FIRSTHDR(msg) is not
null suffices to calm it down, so the strange warnings might have been
the result of an overoptimization based on a supposed UB in the first
place. At least now all versions up to 13.2 as well as clang are happy.