Fix an out-of-bounds write when a zero-length buffer is passed.

[brooks]

Summary

Found with ttyname_test and CHERI bounds checking.

Sponsored by: DARPA, AFRL
Obtained from: CheriBSD

[brooks]

I also wonder if we should perform a NULL pointer check, but POSIX doesn't explicitly allow EINVAL.

[jhb]

In D10377#214967, @brooks wrote:

I also wonder if we should perform a NULL pointer check, but POSIX doesn't explicitly allow EINVAL.

Bruce would argue that a SIGSEGV is a valid NULL pointer check. That is the failure case for many other APIs in userland (e.g. strlen() and strcpy()).