standby server crashes hard on out-of-disk-space in HEAD

logfile from a standby server:

2017-06-12 11:43:46.450 EDT [13605] LOG: started streaming WAL from primary at 3/E6000000 on timeline 1
2017-06-12 11:47:46.992 EDT [11261] FATAL: could not extend file "base/47578/54806": No space left on device
2017-06-12 11:47:46.992 EDT [11261] HINT: Check free disk space.
2017-06-12 11:47:46.992 EDT [11261] CONTEXT: WAL redo at 8/EC7E0CF8 for XLOG/FPI:
2017-06-12 11:47:46.992 EDT [11261] WARNING: buffer refcount leak: [1243] (rel=base/47578/54806, blockNum=5249, flags=0x8a000000, refcount=1 1)
TRAP: FailedAssertion("!(RefCountErrors == 0)", File: "bufmgr.c", Line: 2523)
2017-06-12 11:47:47.567 EDT [11259] LOG: startup process (PID 11261) was terminated by signal 6: Aborted
2017-06-12 11:47:47.567 EDT [11259] LOG: terminating any other active server processes
2017-06-12 11:47:47.584 EDT [11259] LOG: database system is shut down

The FATAL is fine, but we shouldn't have that WARNING I think, and
certainly not the assertion failure.

regards, tom lane