add assertions to isc_buffer macros

if ISC_BUFFER_USEINLINE is defined, then macros are used to implement isc_buffer primitives (isc_buffer_init(), isc_buffer_region(), etc). otherwise, functions are used. previously, only the functions had DbC assertions, which made it possible for coding errors to go undetected. this commit makes the macro versions enforce the same requirements.

Closes #3553 (closed)