MLBEDSW-8575 Tests fails on conv networks

Fixed a problem where the compiler incorrectly called the
mlw_codec to create an empty weight stream for the second
weight core.
Also added code to the mlw_codec to detect this as an value
error rather than a memory error.

Change-Id: I463846cecb1178f8fbf04dc3e39bd6965cb8ddfc
Signed-off-by: Fredrik Svedberg <fredrik.svedberg@arm.com>