kenv: assert that an empty static buffer passed in is "empty"

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).

sys/kern/kern_environment.c

@@ -250,6 +250,8 @@ init_static_kenv(char *buf, size_t len)
 	char *eval;
 
 	KASSERT(!dynamic_kenv, ("kenv: dynamic_kenv already initialized"));
+	KASSERT(len == 0 || *buf == '\0',
+	    ("kenv: sized buffer must be initially empty"));
 
 	/*
 	 * We may be called twice, with the second call needed to relocate