Propose a constant buffer size

[MichaelChirico]

I'm not sure if getting the buffer length precisely correct is crucial for this routine (untested), but I notice the buffer is almost always either size 44-45 so why not just pick one & make things easier?

table(buflen = ceiling(log(2**64 / log(2:36))))
# buflen
# 44 45 
# 32  3

Came across this because the code fails to compile under the -Wvla compiler option. That should be disabled here TBH -- the concern there is getting a stack overflow if the buffer request winds up inordinately large. But in this case the buffer size is very small --> unlikely to cause issues except under extreme memory pressure.

But anyway, we can make things easier on the compiler/etc if we have a constant here which I think should be possible.

To be clear: PR is a suggestion, so feel free to close.