fib: fix debug preprocessor directives

fib: fix debug preprocessor directives

Type: fix

Some debugging declarations were wrapped in an 'ifdef CLIB_DEBUG'.
This seems to always evaluate true because clib.h defines CLIB_DEBUG
to 0 if its not defined.

The result is that if a route table is added and a route is added to
it and then the table is deleted, VPP exits because the debug function
fib_table_assert_empty() gets called whether VPP was started using a
debug build or a release build.

Change the ifdef to 'if CLIB_DEBUG > 0'.

Change-Id: I357dc2c299e81b95244f2f7efaadb8e0de27627a
Signed-off-by: Matthew Smith <mgsmith@netgate.com>