MINOR: dynbuf: make b_alloc() always check if the buffer is allocated

MINOR: dynbuf: make b_alloc() always check if the buffer is allocated

Right now there is a discrepancy beteween b_alloc() and b_allow_margin():
the former forcefully overwrites the target pointer while the latter tests
it and returns it as-is if already allocated.

As a matter of fact, all callers of b_alloc() either preliminary test the
buffer, or assume it's already null.

Let's remove this pain and make the function test the buffer's allocation
before doing it again, and match call places' expectations.