function exists() on paths with variable length has reference leak

[pebbe]

Example:

match (w:word)
where exists( (w)-[:next*2..]->(:word) )
return w;
WARNING:  buffer refcount leak: [412] (rel=base/16384/123554, blockNum=0, flags=0x83000000, refcount=1 1)
WARNING:  relcache reference leak: relation "next_start_idx" not closed

I've also noted the server crashing in cases similar to this.

agens (AgensGraph 2.2devel, based on PostgreSQL 10.4)