mesa: do not throw _mesa_problem when invalid enum is used

Like with other getters, invalid enum is dealt in find_value by setting
error to GL_INVALID_ENUM and returning INVALID_TYPE which makes
get_value_size return 0.

Fixes false 'implementation errors' seen with Piglit test:
   ext_external_objects-memory-object-api-errors

   "Mesa 20.3.0-devel implementation error: invalid value type in GetUnsignedBytei_vEXT()
   Please report at https://gitlab.freedesktop.org/mesa/mesa/-/issues"

v2: add assert to get_value_size() (Lionel)

Fixes: e064d660205 ("mesa: implement glGetUnsignedByte{v|i_v}")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Eleni Maria Stea <estea@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7403>