There should be more information in the postmaster log than you've shown
us, too. The messages you reported are all from backends *other* than
the one that actually crashed. At the very least the log should have
the postmaster's report of an unexpected child death, with a signal
code.
Also, if you can reproducibly provoke the error, try attaching to the
backend process with gdb before you do so. gdb should be able to give a
backtrace from the crash point even if no core file results.
regards, tom lane