test/format can attempt to set an illegal prepare timestamp

[1520055507:682076][37739:0x7efc4f638700], t, WT_SESSION.prepare_transaction: Failed to parse prepare timestamp '0': zero not permitted: Invalid argument

[1520055507:682113][37739:0x7efc4fe39700], t, WT_SESSION.prepare_transaction: Failed to parse prepare timestamp '0': zero not permitted: Invalid argument

############################################

#  RUN PARAMETERS

############################################

abort=0

alter=0

auto_throttle=1

backups=0

bitcnt=4

bloom=1

bloom_bit_count=45

bloom_hash_count=22

bloom_oldest=0

cache=88

cache_minimum=20

checkpoints=on

checkpoint_log_size=102

checkpoint_wait=39

checksum=uncompressed

chunk_size=7

compaction=0

compression=none

data_extend=0

data_source=file

delete_pct=1

dictionary=0

direct_io=0

encryption=none

evict_max=5

file_type=row-store

firstfit=0

huffman_key=0

huffman_value=0

independent_thread_rng=0

in_memory=0

insert_pct=0

internal_key_truncation=1

internal_page_max=11

isolation=random

key_gap=19

key_max=99

key_min=24

leaf_page_max=17

leak_memory=0

logging=0

logging_archive=0

logging_compression=none

logging_file_max=300643

logging_prealloc=1

long_running_txn=0

lsm_worker_threads=3

merge_max=9

mmap=1

modify_pct=98

ops=100000

prefix_compression=1

prefix_compression_min=6

quiet=1

read_pct=1

rebalance=1

repeat_data_pct=39

reverse=0

rows=100000

runs=100

salvage=1

split_pct=58

statistics=1

statistics_server=0

threads=3

timer=360

transaction_timestamps=1

transaction-frequency=55

truncate=1

value_max=1491

value_min=15

verify=1

wiredtiger_config=

write_pct=0

############################################

t: FAILED: ops/1065: ret == 0 || ret == WT_PREPARE_CONFLICT

process aborting

[1520055507:682176][37739:0x7efc5063a700], t, WT_SESSION.prepare_transaction: Failed to parse prepare timestamp '0': zero not permitted: Invalid argument

*** Aborted