bhyve: avoid a potential deadlock

This unlocks a mutex in an error path, that would otherwise remain locked and
potentially cause a deadlock later on.

Reported by:		Coverity Scan
CID:			1521334
Reviewed by:		corvink, markj
MFC after:		1 week
Sponsored by: 		The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D45401

usr.sbin/bhyve/tpm_intf_crb.c

@@ -348,8 +348,10 @@ tpm_crb_mem_handler(struct vcpu *vcpu __unused, const int dir,
 			pthread_mutex_lock(&crb->mutex);
 			tpm_crb_mmiocpy(&start, val, size);
 
-			if (!start.start || crb->regs.ctrl_start.start)
+			if (!start.start || crb->regs.ctrl_start.start) {
+				pthread_mutex_unlock(&crb->mutex);
 				break;
+			}
 
 			crb->regs.ctrl_start.start = true;