If this format ever happens, it is probably left-aligned.

(Having unused bytes in the left/low/front makes no sense.)
Remove obsolete comment.
Fix the PEEK_INFNAN (wrong macro arg name).

Also rename as INFNAN_PEEK, to match HEXFP_PEEK.

Add "send" pointer to INFNAN_PEEK to guard againt past-the-buffer peeking.

HEXFP_PEEK doesn't easily lend itself to "send" pointer
because of it's calling environment doesn't have one.