COMP: silence warning of vnl_vector_fixed_ref::assert_size

When compiled in release mode assert macro is empty,
so sz parameter is unused.  Added #if !defined(NDEBUG)
bracket around function to silence warning.

Change-Id: Idd5bacfaee3fa17642fd1c189edd32cb6401721d
Merge topic 'SilenceVnlWarning'

f9c1e120 COMP: silence warning of vnl_vector_fixed_ref::assert_size