fib: fix coverity 249175

Add an assert to express the constraint to coverity without
incurring the overhead in release builds.

Type: fix
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: I2c22f8b2565c645d95c9c0be37381060e151420f
diff --git a/src/vnet/fib/fib_entry_src.c b/src/vnet/fib/fib_entry_src.c
index c0275e8..39e719e 100644
--- a/src/vnet/fib/fib_entry_src.c
+++ b/src/vnet/fib/fib_entry_src.c
@@ -46,6 +46,7 @@
         return (&fib_entry_src_bh_vft[FIB_SOURCE_BH_INTERPOSE]);
     }
 
+    ASSERT(bh < FIB_SOURCE_BH_MAX);
     return (&fib_entry_src_bh_vft[bh]);
 }