Set the 'inst_length' to '0' early on before any error conditions are detected
in the emulation of the task switch. If any exceptions are triggered then the
guest %rip should point to instruction that caused the task switch as opposed
to the one after it.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270855 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f