Make assertion explicit.
Make assertion explicit.

This makes it easy to reason why we can do mem[EI_MAG$n] without going
out of bounds.

Change-Id: Ibeaf5a361320f46265d16c21f05759f85a721ffc
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto