app-layer: Set sc_errno upon error return

app-layer: Set sc_errno upon error return

Bug: https://redmine.openinfosecfoundation.org/issues/6782

Callers to these allocators often use ``sc_errno`` to provide context of
the error. And in the case of the above bug, they return ``sc_errno``,
but as it has not been set ``sc_errno = 0; == SC_OK``.

This patch simply sets this variable to ensure there is context provided
upon error.

(cherry picked from commit fc2e49f84adcd471ca70c8e6aa82e2cf9f7b17ad)