exec.library: The start of a memory region may not be a 'VALID_PTR'

exec.library: The start of a memory region may not be a 'VALID_PTR'

The MemHeader may sometimes *not* be referenced by another memory
region - for example, if the MemHeader is at the start of the
memory region it is mapping, then mh_Lower is usually set to:

AllocMemHeader() {
...
mh->mh_Lower = (APTR)mh + MEMHEADER_TOTAL
...
}

If this is the first memory region, this will also be the case.

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@47448 fb15a70f-31f2-0310-bbcc-cdcc74a49acc