In store.c :67 libcouchbase_store_by_key does not check for negative value for idx, causing seg fault when nodes are down

I had 3 server nodes, 2 of which were down.

When I ran my test program (to store a value), instead of reporting an error or using the remaining node, the program seg faulted.
I traced this back to store.c :66, where vbucket_map(instance->vbucket_config, hashkey, nhashkey, &vb, &idx)
returned -1 for the value idx (the index into the vbucket); this probably indicates that there was no valid index.
The next line, 67, uses idx to reference a server struct. Using -1 causes an invalid memory access and the program crashes with a seg fault.

server = instance->servers + (size_t)idx;

There should be a statement to check the value idx is non negative before using it: