refactor(el3-spmc): crash instead of reading OOB

If it is called on an invalid mtd, out-of-bounds memory reads are
likely.  Checks elsewhere in the code ensure that the mtd has been
validated before calling this function.

Change-Id: If598680a5b79e1786a6e0a213779ec80cbf37494
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
1 file changed