sanitycheck: when present, point at handler.log instead of run.log

When Zephyr crashes immediately QEMU reports an error immediately. This
is immediately reported by "make run". Then sanitycheck points the user
at the output of "make run". However the error message(s) are in QEMU's
output which is in a different .log file.

To address this situation point the error message at handler.log
instead of run.log if and only if handler.log is not empty.

To reproduce here's an artificial but very simple crash:

  sanitycheck --extra-args=CONFIG_TEST_USERSPACE=n \
    -p qemu_x86 -T tests/kernel/mem_protect/stackprot/

Signed-off-by: Marc Herbert <marc.herbert@intel.com>