spawn (Inline::C): fix off-by-one error

Noticed while testing on FreeBSD 11.2 amd64 with the optional
Inline::C extension using clang 6.0.0.  The end result on
FreeBSD was spawning processes failed badly and things were
immediately unusable with this enabled.

av_len is a misleading API, and I failed to read the API
comments in perl:/av.c which state:

> Note that, unlike what the name implies, it returns
> the highest index in the array, so to get the size of
> the array you need to use "av_len(av) + 1".
> This is unlike "sv_len", which returns what you would expect.

If this bug affected anybody, it would've only affected users
using both the optional Inline::C module AND set the
PERL_INLINE_DIRECTORY environment variable.

That said, I've never seen any evidence of it on Debian
GNU/Linux + gcc on any x86 variant.  That includes full 64-bit
systems, a full 32-bit system, a 64-bit system with 32-bit
userspace, across multiple gcc versions since 2016.

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