rusticl/api: Fix creating a program if a nul byte is within the specified string...

rusticl/api: Fix creating a program if a nul byte is within the specified string length

The code assumed that if the length of a string was specified and greater than zero, the string
would not contain a nul byte.
Legal or not, there are apparently applications which violate that assumption. Since the spec
doesn't say anything about this case, take the likely most compatible route and treat a nul byte as
terminating the string early.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7408
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18978>