CPP11 — sequence allocbuf parameter != 0

Summary: Sequence allocbuf functions should be specified such that passing zero for
their arguments is illegal. Allocating a sequence buffer of zero elements
is rather pointless because nothing useful can be done with such a buffer.
The result of passing zero to allocbuf should be implementation-defined,
since throwing an exception is not a good way to handle programming logic
errors (similar to how indexing past the end of a sequence is a logic error
for which an exception is pretty useless).

Close no change. Discussion of this issue determined that a zero parameter to allocbuf is legal and