Fixed raw-tag-access not working

[Naarakah]

I have fixed a mistake that made raw-tag-access unusable.

[felixc]

Hello again! Thank you very much for the fix!

For my own education, and so I can put it in the commit message for future reference, could you describe the problem and the fix? I'm not familiar enough with Rust FFI to understand the change.

[Naarakah]

A null pointer was passed to glib_sys::g_bytes_get_data as the argument supposed to be used to return the data size, as I initially thought it was possible (because there's another function to get that size), but the C function behind glib_sys::g_bytes_get_data crashes when given a null pointer so I changed that.

[felixc]

Hey! Sorry for the long delay, but this finally landed in f6c90e7.

GitHub doesn't show this PR's status as "merged" because I had to rebase onto some other commits (and I added a test), so I'll have to "close" this — but the change is indeed in master and your contribution is very much appreciated!