radeonsi: verify buffer_offset value before using it

radeonsi: verify buffer_offset value before using it

This buffer_ofset can come directly from the application (e.g: when using
glVertexAttribPointer) and can contain an invalid value.

st_atom_array already makes sure that if it's not negative so all that's left
is to verify that it's smaller that the buffer size.

Bugs related to this issue:

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105251#c52
Bugzilla: https://bugzilla.freedesktop.org/show_bug.cgi?id=109693
Signed-off-by: Marek Olšák <marek.olsak@amd.com>