bhyve: Don't free an invalid pointer.

The netmap-specific data stored at be->opaque is freed by the caller
on error as part of freeing be.

Reviewed by: markj
Reported by: GCC -Wfree-nonheap-object
Differential Revision: https://reviews.freebsd.org/D36828

(cherry picked from commit 2fb81691b02f27fa938f311835a8c6704c5cf80f)