DEBUG: buf: add BUG_ON_HOT() to most buffer management functions

DEBUG: buf: add BUG_ON_HOT() to most buffer management functions

A number of tests are now performed in low-level buffer management
functions to verify that we're not appending data to a full buffer
for example, or that the buffer passed in argument is consistent in
that its data don't outweigh its size. The few functions that already
involve memcpy() or memmove() instead got a BUG_ON() that will always
be enabled, since the overhead remains minimalist.