Make clearer that size = 0 in allocate_field() is not allowed.

Back in design phase the code used realloc() for freeing the memory
also. However, this is not entirely portable, and therefore the finished
implementation used free() separately.

There were some remnants of the size = 0 code in the allocate_field()
code, which made it somewhat confusing. This change makes it clearer
that size = 0 is not allowed (and not used by nanopb).