Remove some alloca uses

A few spots (mostly in the parsers) use alloca to ensure that a string
is terminated before passing it to a printf-like function (mostly
'error').  However, this isn't needed as the "%.*s" format can be used
instead.

This patch makes this change.

In one spot the alloca is dead code and is simply removed.

Regression tested on x86-64 Fedora 38.

Approved-By: John Baldwin <jhb@FreeBSD.org>