add va_end() calls where missing for a va_start() or va_end().

Fix for Coverity perl5 CIDs 29225, 29226, 29227, 29228, 29229: Missing
varargs init or cleanup (VARARGS) missing va_end: va_end was not
called for foo.

Use of va_args must be finished off with va_end (in other words,
use of va_start or va_copy must be bracketed off with va_end).
In most platforms va_end is a no-op, but in some platforms it is
required for proper cleanup (or face stack smash, or memory leak).

Tony: move va_start() out of the declaration block
bump $PerlIO::via::VERSION to 0.15