Garbage in the passed-in buffer can cause problems if any attempts to read
the kenv are inadvertently made between init_static_kenv and the first
kern_setenv -- assuming there is one.
This is cheap and easy, so do it. This also helps rule out some class of
bugs as one tries to debug; tunables fetch from the static environment up
until SI_SUB_KMEM + 1, and many of these buffers are global ~4k buffers that
rely on BSS clearing while others just grab a page of free memory and use it
(e.g. xen).
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