fib: add invalid source type and fix debug log

Add the FIB_SOURCE_INVALID fib source type. This allows to spot
uninitialized fib source more easily (0 no longer means special) and we
can use it as placeholder when no source is present.
Use it to fix FIB_ENTRY_DBG() which was accessing the 1st source, even
when no sources were present.

Type: fix
Fixes: 710071bf0e

Change-Id: I980b6a6a07616d4a8d6f2db166a1dd335721c74d
Signed-off-by: Benoît Ganne <bganne@cisco.com>