fib: Crash when specify a big prefix length from CLI.

The VPP is crashing when specify a very big prefix length, like
ip route add 1.1.1.1/55 via 2.2.2.2

Type: fix

Signed-off-by: Gavril Florian <gflorian@3nets.io>
Change-Id: Ic491c0b24e07be897ff35ae1e835280f04ab3ea5
diff --git a/src/vnet/error.h b/src/vnet/error.h
index 3feb8ff..fa13375 100644
--- a/src/vnet/error.h
+++ b/src/vnet/error.h
@@ -158,7 +158,8 @@
   _ (BUSY, -167, "Busy")                                                      \
   _ (BUG, -168, "Bug")                                                        \
   _ (FEATURE_ALREADY_DISABLED, -169, "Feature already disabled")              \
-  _ (FEATURE_ALREADY_ENABLED, -170, "Feature already enabled")
+  _ (FEATURE_ALREADY_ENABLED, -170, "Feature already enabled")                \
+  _ (INVALID_PREFIX_LENGTH, -171, "Invalid prefix length")
 
 typedef enum
 {