Fix: initialize parallel_workers to zero for CdbPathLocus_HashedOJ

For test case:

create table t0(c0 inet) distributed randomly;
create table t2(c0 inet) distributed randomly;
create table t3(c0 inet) distributed randomly;

SELECT ALL t2.c0, t3.c0, t0.c0 FROM t0, ONLY t3 FULL OUTER JOIN t2 ON ((t2.c0)=(t3.c0))
WHERE (((('0.5496844753539182')||(t3.c0)))LIKE(CAST((0.13292931)::MONEY AS VARCHAR(971))))
UNION ALL SELECT t2.c0, t3.c0, t0.c0 FROM t0, ONLY t3 FULL OUTER JOIN t2 ON ((t2.c0)=(t3.c0))
WHERE NOT ((((('0.5496844753539182')||(t3.c0)))LIKE((CAST(0.13292931 AS MONEY))::VARCHAR(971))))
UNION ALL SELECT ALL t2.c0, t3.c0, t0.c0 FROM t0*, ONLY t3 FULL OUTER JOIN t2 ON ((t2.c0)=(t3.c0))
WHERE ((((('0.5496844753539182')||(t3.c0)))LIKE((CAST(0.13292931 AS MONEY))::VARCHAR(971)))) ISNULL;

will cause crash because of assert failure in 'create_plan_recurse'.

'apache#3  0x00007fe94eccf476 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
 apache#4  0x00007fe94ecb57f3 in __GI_abort () at ./stdlib/abort.c:79
 apache#5  0x00007fe94fcdd548 in ExceptionalCondition (conditionName=0x7fe95043dcd0 "best_path->parallel_workers == best_path->locus.parallel_workers",
     errorType=0x7fe95043db06 "FailedAssertion", fileName=0x7fe95043dbdb "createplan.c", lineNumber=623) at assert.c:48
 apache#6  0x00007fe94f94918f in create_plan_recurse (root=0x55d7cbe96f78, best_path=0x55d7cbec0380, flags=1) at createplan.c:623
 apache#7  0x00007fe94f94a1f8 in create_append_plan (root=0x55d7cbe96f78, best_path=0x55d7cbec0700, flags=1) at createplan.c:1380
 apache#8  0x00007fe94f948d37 in create_plan_recurse (root=0x55d7cbe96f78, best_path=0x55d7cbec0700, flags=1) at createplan.c:481
 apache#9  0x00007fe94f94e2d1 in create_motion_plan (root=0x55d7cbe96f78, path=0x55d7cbec0e50) at createplan.c:3316
 #10 0x00007fe94f9490dc in create_plan_recurse (root=0x55d7cbe96f78, best_path=0x55d7cbec0e50, flags=1) at createplan.c:608
 apache#11 0x00007fe94f948ba3 in create_plan (root=0x55d7cbe96f78, best_path=0x55d7cbec0e50, curSlice=0x55d7cbe96f20) at createplan.c:392'

The parallel_workers should be set to zero because parallel full join is not supported yet.

src/include/cdb/cdbpathlocus.h

@@ -299,6 +299,7 @@ typedef struct CdbPathLocus
         _locus->locustype = CdbLocusType_HashedOJ;		\
         _locus->numsegments = (numsegments_);           \
         _locus->distkey = (distkey_);					\
+        _locus->parallel_workers = 0;                   \
         Assert(cdbpathlocus_is_valid(*_locus));         \
     } while (0)
 #define CdbPathLocus_MakeHashedWorkers(plocus, distkey_, numsegments_, parallel_workers_)      \