bhyve: rfb: do not destroy a mutex that was never initialized

[wanpengqian_gmail.com]

Summary

rfb_init()'s error path tests pixfmt_mtx against NULL, which only
compiles where pthread_mutex_t is a pointer, and destroys it. Every
jump to that label is before the mutex is initialized, so there is
nothing to destroy. The test and the destroy came in with the
SetPixelFormat support (D46402).

Fixes: dda0f9837b1c ("bhyve: support noVNC SetPixelFormat request")
Signed-off-by: Wanpeng Qian <[e-mail]>
Sponsored by: keelos.dev

Test Plan

Built usr.sbin/bhyve on main 7cb4d4dfd with and without the change, plain
and WITH_BHYVE_SNAPSHOT, -Werror: 0 warnings.

Every "goto error" in rfb_init() comes before pthread_mutex_init(
&rc->pixfmt_mtx), so the error path never has a mutex to destroy.

Error path exercised: with 127.0.0.1:5999 already taken, bhyve with
-s 29,fbuf,tcp=127.0.0.1:5999 exits with status 4 and "bind: Address
already in use / Device emulation initialization error" in both versions.