ping: Fix coverity error 163907

Return an error if the ip6_interface_first_address decides to return NULL.

Change-Id: Iea0184382b8fbe2af81a781c3671687a1fbb5b32
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
diff --git a/src/vnet/ip/ping.h b/src/vnet/ip/ping.h
index 1887314..0af9d58 100644
--- a/src/vnet/ip/ping.h
+++ b/src/vnet/ip/ping.h
@@ -32,6 +32,7 @@
   SEND_PING_ALLOC_FAIL,
   SEND_PING_NO_INTERFACE,
   SEND_PING_NO_TABLE,
+  SEND_PING_NO_SRC_ADDRESS,
 } send_ip46_ping_result_t;
 
 /*