Fixes to ShadingSystemImpl::decode_connected_param

[lgritz]

@marsupial correctly points out that we are unsafely passing
a string_view.data() into both atoi and strchr, which is unsafe because
there's no guarantee that a string_view is null-terminated. Ugh!

He proposed a fix here: #800
and there's nothing wrong with it per se, but I suspected it could be done
more simply, so this PR is my stab at it, using the OIIO::Strutil::parse_*
functions.

[lgritz]

This is a counter-proposal to #800

[fpsunflower]

LGTM

[marsupial]

Seems to play nicely here with #801.

FWIW OIIO::Strutil::parse_int also looks to have the same issue and may parse beyond the string_view's length.
line 689: p.remove_prefix (end-p.begin()); can throw with a std::string_view.

[lgritz]

Oh, let me have a look at parse_int, too, then.

[lgritz]

Ow, you are quite right. Will make a fix there...

[lgritz]

parse_float as well...

[lgritz]

I want to point out that this is a legit bug according to spec, but in practice in our code base the string_view's are always generated from a std::string or ustring or null-terminated char* (or a substring of one of those), so it's not a huge emergency "how did this ever work" kind of bug -- in our code, every string_view does point to (or within) a null-terminated string. So it can't crash, but it can generate wrong results if there's a string like "1234" and the string_view is referring to just the first 2 characters (should parse to 12, but will erroneously parse to 1234). I don't think we actually do this anywhere, but it's a nasty bug just waiting to happen, so I want to fix it.

[lgritz]

I feel like this issue is very much related to the changes in AcademySoftwareFoundation/OpenImageIO#1785, so I think I'll address the primary problems with the parse functionality there.

[marsupial]

The strchr bug was definitely creating bad results for me.

I agree the rest is a bit pedantic from what I can see in the context of OSL, but it's not too hard to imagine an exploit where OIIO (or some client of) tries parsing a well crafted string in some image header/metadata.

[lgritz]

I agree totally! I'm working on a robust fix for it all.