MLBEDSW-9868: Adjust BufferReader asserts to always allow index 0
Index 0 is unstrided so we can always allow reading that index regardless of stride. Also added a few more asserts.
Signed-off-by: Johan Gunnarsson johan.gunnarsson@arm.com Change-Id: Icf7ac8c46665d4e651232d0c0ba66733dce99dc6