bt: avoid unintended sign extension

bt: avoid unintended sign extension

In the case of a 4-byte length, shifting a value by 24 may cause
an unintended sign extension when converting from int to size_t.
Use a uint32_t variable instead.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>