Remove magic numbers in codec functions

[jmcph4]

Often, as a quick validation check, the length of a az_bytearray_t will be checked to ensure it's at least three bytes long, due to inference from the actual multiformat specifications themselves. While this is technically valid, the current codebase is now littered with nasty magic numbers.

These belong in a macro constant of some sort, perhaps defined in src/azorian.h.